BLyS drives affinity maturation

I have found an interesting paper that describes in the very detailed way how the interactions between two specialized subsets of lymphocytes which are germinal center (GC) B cells and follicular helper (FH) T cells may influence the affinity maturation of antibodies. The cooperation of these two subsets is known for the long time. However, the novelty this publication brings about is that it looks specifically at the germinal center reaction and dissects the role that the molecule called BLyS (or interchangeably BAFF) may play in the affinity maturation of antibodies from other tasks fulfilled by the same protein. In other words, BLyS has important functions in many aspects of B cells life and the exact definition of its role in the affinity maturation process was not possible using relatively straightforward methods like studying mice with deletion of gene encoding BLyS.

The link: http://jem.rupress.org/content/211/1/45.abstract

Authors perform the study on mice that were immunized with nitrophenacethyl hapten conjugated with chicken gamma globulin which is a widely used method to elicit a strong germinal center reaction. What forms the basis of this paper is the observation that receptor-bound BLyS seems to be selectively excluded form B cells that are located inside germinal centers. Investigators follow with the demonstration that activated B cells downregulate TACI (one of three BLyS receptors), that IL-21 may be responsible for the observed TACI downregulation and finally that in the context of germinal center (and unlike systemically) the main source of BLyS comes from cells of hematopoietic origin, namely follicular helper T cells.

But what role the T cell-derived BLyS may have for the quality of the antibody response? To answer this question authors create an experimental setting (mixed bone marrow chimeras) where the T cell lineage lacks BLyS. In such circumstances germinal centers do form and are maintained mostly normally. However, the ability to form high-affinity antibodies is visibly impaired when germinal centers operate without T-cell derived BLyS.

Radhika Goenka, Andrew H. Matthews, Bochao Zhang, Patrick J. O’Neill, Jean L. Scholz, Thi-Sau Migone, Warren J. Leonard, William Stohl, Uri Hershberg, and Michael P. Cancro (2014). Local BLyS production by T follicular cells mediates retention of high affinity B cells during affinity maturation Journal of Experimental Medicine DOI: 10.1084/jem.20130505

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Observations on the B cell repertoire in young and elderly people

Since I have not written for a while I decided to choose for my comeback something that could be summarized in couple of shorts paragraph, something that I am not very familiar with, so I am not grounded in endless divagations yet something that is of enough interest to whet my appetite for more posts to come soon. I selected an article that compares the overall B cell repertoires between humans that are respectively young or elderly as well as brings an additional variable to the age parameter, that is the seropositivity for either CMV or EBV.

The link: http://www.jimmunol.org/content/192/2/603

This is a study in which authors analyze the rearanged heavy chain gene sequences from PMBC cells isolated from peripheral blood of study participants that are assigned to three different age group. Additionally all collected samples were assessed for the presence of anti-CMV or EBV antibodies. The conclusions could be put in a nutshell in few sentences. There seems to be no difference in V, D and J usage between young and elderly age groups. However, the older age appears to correlate with lengthening of the CDR3 region. People advanced in years also harbor more highly mutated IgM and IgG Ig genes and some of them display a trend towards the accumulation of expanded and persistent B cell clones. Last but not least, either the chronic infection with CMV or EBV appears to imprint its own discreet mark on the overall B cell repertoire.

Krishna M. Roskin, Tho D. Pham, Jonathan Laserson, Chen Wang, Yi Liu, Lan T. Xu, Katherine J. L. Jackson, Eleanor L. Marshall, Katie Seo, Ji-Yeun Lee, David Furman, Daphne Koller, Cornelia L. Dekker, Mark M. Davis, Andrew Z. Fire and Scott D. Boyd (2014). Effects of Aging, Cytomegalovirus Infection, and EBV Infection on Human B Cell Repertoires. Journal of Immunology DOI: 10.4049/jimmunol.1301384

On the wild mice and autoimmunity

Only the relatively small part of mammalian genome is formed by protein-coding sequences. The long stretches between these protein-bearing fragments contain other components which could be either non-transcribed regulatory elements or the variety of non-translated RNAs many of which are also taking part in the regulation of gene expression. This intricate network influences the decision whether a given protein is present in the particular physiological situation and how much of it is available. Therefore the issue how proteins are regulated may in fact be equally or more important than the structural differences in coding parts of the protein in question. The problem of transcriptional regulation that affects the autoimmunity development composes the leading theme of the publication I am discussing today. Like the majority of experimental studies this work uses the cornerstone model organism – the mouse Mus musculus. However, unlike the most it provides also some glimpse into the natural populations of rodents.

The link: http://jem.rupress.org/content/209/12/2307.abstract

The particular problem analyzed by this publication comprises the association between FcγRIIb receptor and the aptitude to develop autoimmune reactions. Authors analyze the populations of wild mice from various part of the globe and find out that the vast majority of them are in the possession of FcγRIIb haplotype that is also present in several laboratory mice known for their propensity to develop the autoimmunity. Such arrangement is of interest because it suggests that in the wild populations such autoimmunity-correlated variant of FcγRIIb may actually be positively selected.

FcγRIIb is the receptor that recognizes the constant portion of an antibody that has switched to the IgG isotype and unlike other receptors that also bind to IgG its ligation on B cells causes the modulation of immune responses. To gain an insight of how FcγRIIb haplotype which is predominantly present in the wild mice may influence the immune response of the classic laboratory strain C57BL/6 (which itself has FcγRIIb haplotype not associated with the autoimmunity) investigators exchange the copies of that receptor by the knock-in approach and study the immune parameters of the resulting strain.

It turns out that the alterations between two haplotypes lie in their transcriptional regulation. In the non-autoimmune setting (as demonstrated by C57B/6 strain) FcγRIIb is upregulated on germinal center B cells whereas the knocked-in variant of this receptor cancels this up-regulation. Additionally, when compared to to C57BL/6 line the novel knock-in strain displays the reduced amount of FcγRIIb on activated B cells, splenic transitional B cells and bone marrow residing pre-B cells. Authors pin down the difference in the DNA sequence that underlies the disparate regulation of two variants and propose the transcription factor which may be responsible for the up-regulation seen when the non-autoimmune haplotype is present. Finally, investigators also show that the two haplotypes in questions differ in qualitative terms that comprise the number of germinal center B cells, affinity maturation and autoimmunity development.

Why do I think that this report is interesting? When you look at the paper conclusions from the broader perspective they seem to confirm the notion that in the natural circumstances the autoimmunity is not a problem. The widespread presence of that particular FcγRIIb haplotype among the wild mice suggests that at least for germinal center B cells the natural selection may have favored the situation where the efficiency of immune reaction is maximized even at the cost of potential collateral damage. Maybe it looks like the obvious notion but it is good to have some hard data that confirm it.

Espeli, M., Clatworthy, M., Bokers, S., Lawlor, K., Cutler, A., Kontgen, F., Lyons, P., & Smith, K. (2012). Analysis of a wild mouse promoter variant reveals a novel role for Fc RIIb in the control of the germinal center and autoimmunity Journal of Experimental Medicine, 209 (12), 2307-2319 DOI: 10.1084/jem.20121752

PAX5 ability to repress BLIMP1 is phosphorylation-dependent

When a B cell starts releasing antibodies on the large scale it undergoes the sequence of deep morphological and physiological changes. It expands its cytoplasm and switches on the machinery that lets it cope with the enhanced protein production in preparation to professional antibody secretion. On the molecular level this transition it governed by the onset in expression of certain transcription factors whereas other transcription factors are being switched off. The main players in the transformation of a B cell to a plasma cell are transcription factors PAX5 and BLIMP1. PAX5 is expressed in all B cells except the plasma cell stage while BLIMP1 holds the position of chief regulator that opens the way to antibody secreting cell phenotype. Additionally, it is known that PAX5 represses BLIMP1 and that this repression can be relieved by signaling through BCR. I have read the publication which examines details of the interactions between PAX5, BLIMP1 and the signaling relay activated by BCR stimulation.

The link: http://www.jimmunol.org/content/188/12/6127.abstract

The report’s leading theme is the analysis how the phosphorylation status of PAX5 may influence its ability to repress BLIMP1 promoter. Initially authors identify two residues in PAX5 sequence that can be phosphorylated by ERK kinase which is a part of cascade that transmits the signal originating from BCR. Since the repression competence of PAX5 may be dictated by its phosphorylation state investigators test whether it is possible to activate Blimp-1 promoter when PAX5 is modified to become unresponsive to ERK-driven phosphorylation. To this end they demonstrate that the BCR activation starts off the signaling cascade which terminates with PAX5 phosphorylation by ERK and that the phosphorylated PAX5 is unable to maintain the repression of BLIMP1 promoter. However, the mutated form of PAX5 with its phosphorylation sites replaced by alanine substitutions continues the transcriptional repression of Blimp-1 regardless of BCR activation. Thus the signal at BCR may initiate the plasmacytic differentiation because it changes PAX5 phosphorylation status.

The discussed publication studies the molecular events only and due to a number of technical limitations (see the discussion chapter in the original paper) it does not attempt to translate its findings to the actual immune response (e.g. whether the same PAX5 modifications may influence plasma cell counts). Why do I think it is interesting, though? The answer is I have just read another paper published by Mark Shlomchik’s group (B Cell Receptor Signal Transduction in the GC Is Short-Circuited by High Phosphatase Activity. Science; 2012. 336: 1178-81). That paper examines the extend of BCR signaling during the germinal center reaction and finds it very limited.

According to authors in GC B cells the phosphorylation level of several components belonging to BCR signaling cascade is diminished compared to non-GC B cells. Such reduction is due to the enhanced phosphatase activity which abolishes signals that could be potentially transmitted downstream when BCR is activated during germinal center reaction. Moreover, the specific to B cells inactivation of SHP-1 phosphatase decreases the GC B cell frequency which indicates that the temporal inhibition of BCR signaling may be actually vital for the proper GC maintenance.

The key findings of these two papers (1) The signaling at BCR may open the way to plasma cell phenotype through phosphorylation cascade which completes with PAX5-mediated relieve of BLIMP1 repression. (2) In GC B cells BCR signaling is limited because of high phosphatase activity make a lot sense when combined together. Germinal centers are anatomical sites where B cells undergo antigen-driven affinity maturation and the GC reaction which is terminated too early would compromise the quality of immune response. It looks like the stimulation at BCR may be able to set off the change in the transcription factor architecture (through the phosphorylation-dependent mechanism) that could promote the immediate plasmacytic differentiation. Therefore the temporal modulation of BCR signaling during GC reaction could be crucial because it would ensure enough time to generate high affinity antibody response. Is this the case? Maybe it is.

Yasuda T, Hayakawa F, Kurahashi S, Sugimoto K, Minami Y, Tomita A, & Naoe T (2012). B cell receptor-ERK1/2 signal cancels PAX5-dependent repression of BLIMP1 through PAX5 phosphorylation: a mechanism of antigen-triggering plasma cell differentiation. Journal of immunology (Baltimore, Md. : 1950), 188 (12), 6127-34 PMID: 22593617

Borrelia burgdorferi – the master manipulator

Who are the most accomplished immunologists in the world? The title may go to several pathogenic organisms that are apparently able to manipulate immune responses and do it in the way that puzzles many researchers. Bacterium Borrelia burgdorferi (the causative agent of Lyme disease) definitely belongs to the elite club. I have learnt that during infection it does not even try to hide away and assumes distinctively bold tactics as it migrates to the very hub of protective action – the draining lymph node. And there it does not sit quietly either since it can cue B cells to what it looks like the unusual (plus yet unexplained) proliferation which probably hinders the quality of ensuing protective response.

The link: http://www.jimmunol.org/content/188/11/5612.abstract

The discussed paper is a continuation of the report which was published by the same group last year (Lymphoadenopathy during lyme borreliosis is caused by spirochete migration-induced specific B cell activation. PLoS Pathog. 7: e1002066). Since I think it is important to combine the information from both papers I am going to summarize shortly the findings of that first publication before moving on to more recent results. Authors observed that when they infected mice with Borrelia using the natural route (tick’s bite) sick animals displayed the substantial enlargement of lymph nodes that were most adjacent to bite locations. In order to control the actual site of infection (ticks are living animals and they can move freely before starting their blood meal) as well as to avoid the direct use of culture-grown bacteria (which may stimulate the different type of immune response than bacteria from infested ticks) investigators have devised a modified infection procedure. Shortly, they injected immunocompromised mice (SCID) with culture-grown Borrelia and transplanted biopsies from such infected animals into the right tarsal joint of naive mice. This innovation has allowed focusing on the single draining lymph node while it exposed animals to host-adapted bacteria.

The particular problem that authors have tackled was how the Borrelia infection altered the right inguinal lymph node and whether there were any further modifications to the lymphatic architecture as the disease progressed. Investigators confirmed the rapid and intense accumulation of B cells in the draining lymph node and also noticed that this accumulation subsequently spread to more distant lymph nodes but not to the spleen. Such ensuing B cell response was critically dependent on the presence of live bacteria inside the lymph node yet quite surprisingly it occurred without any perturbations in the absence of MyD88. Apart from that, authors demonstrated that the immune reaction going on in affected lymph nodes was at least partially specific to Borrelia antigens.

In the follow-up paper researchers attempt to answer the question what is the role of CD4 T cells in the B cell accumulation prompted by Borrelia infection. They find out that CD4 T cells from affected lymph nodes do not increase their numbers as it happens to B cells yet they become activated along the course of disease. Nevertheless, the B cell buildup takes place without CD4 T cells as it did without MyD88. The anti-Borrelia antibody response, however, is weaker when there are no CD4 T cells around.

The overall picture of the immune response to Borrelia in the model that uses host-adapted bacteria (which mimics the natural infection) looks somehow paradoxical and misshapen. First pathogens invade the closest lymph node and seem to provoke there the massive B cell proliferation which disperses later to other lymph nodes. This proliferation is independent of mitogenic cues imparted by TLR signaling and it happens without CD4 T cell-driven costimulation as well. The specific anti-Borrelia antibody response (partially dependent on CD4 T cells) is then switched on but it gives the impression of being not completely normal, too. Authors show that the germinal center induction in lymph nodes is delayed and all germinal centers tend to decline very rapidly. However, plasma cells (which are thought to derive from the germinal center reaction) accumulate with kinetics suggesting that they are not generated in germinal centers located in lymph nodes. Investigators postulate that these plasma cells may originate from ectopic lymphoid tissues.

But it is the initial B cell accumulation that probably distorts the quality of anti-Borrelia immune response. Authors present data showing that this accumulation is indeed able to destroy the inherent organization of an affected lymph node. The question that I have is whether it happens because of sheer number of B cells or maybe through some defined B cell-specific antibody-independent effector mechanism like for example the release of a chemokine that interferes with the layout of a lymph node. Another interesting enigma is how Borrelia targets B cells and what receptor on B cell surface intercepts the signal.

Hastey CJ, Elsner RA, Barthold SW, & Baumgarth N (2012). Delays and diversions mark the development of B cell responses to Borrelia burgdorferi infection. Journal of immunology (Baltimore, Md. : 1950), 188 (11), 5612-22 PMID: 22547698

B cells can secrete IL-6 and drive Th17 response in autoimmunity

The main task of B cells is to release protective immunoglobulins. Yet it is not their only role since they are apparently capable to take on the diverse array of activities that do not directly form a part of effector humoral responses. Instead of just secreting antibodies B cells can influence the outcome of an immune response by dictating the behavior of other cell types. It appears that such mechanism may underlie the development of autoimmunity in the central nervous system. I have found the publication that by combining the work on a mouse experimental model and the analysis of human patients presents the compelling evidence pointing to how B cells stimulate CD4 T cells into the pathogenic phenotype through the antibody-independent action.

The link: http://jem.rupress.org/content/209/5/1001.abstract

The aim of the paper is to seek a clarification for the unexplained outcomes of various B cell depletion treatments during the course of multiple sclerosis or its mouse model – EAE. For example, the targeted reduction of antibody-secreting plasma cells results in the worsening of disease symptoms. Similarly, the broad B cell depletion leads to the improvement that precedes the drop in the level of autoantibodies. To understand whether B cells may have an additional antibody-independent role in the pathogenesis of multiple sclerosis or EAE investigators focus on IL-6 which is a pro-inflammatory cytokine and the essential factor in autoimmune conditions that afflict the central nervous system. They analyze how B cells can contribute to IL-6 secretion in the context of autoimmunity and assess if B cell-derived IL-6 may influence the conduct of CD4 T cells which are the main factor in multiple sclerosis/EAE development.

Authors show that B cells have the inherent ability to secrete IL-6 when they are stimulated with ligands engaging innate receptors (LPS or CpG plus anti-CD40 antibody) and attempt to prove that such B cells’ capacity may have the physiological importance.  To this end they demonstrate that B cells collected from mice that received EAE-driving immunizations are enriched for IL-6 mRNA compared to B cells from healthy mice. Additionally, the abrogation of IL-6 expression in B cells blunts the severity of neurological symptoms typical to EAE. The final proof indicating for the antibody-independent role of B cells in murine EAE pathogenesis comes from BCDT (B cell depletion therapy) experiments as such treatment is effective in alleviating the EAE-resulting damage only when B cells are competent to release IL-6. On the other hand, knocking out IL-6 expression in B cells does not influence their capacity to secrete antibodies. Investigators also pinpoint that marginal zone B cells are the subset which is most proficient in IL-6 release.

IL-6 is a cytokine involved in the formation of Th17 subset which is known for its participation in pathogenic responses pertinent to the autoimmunity driven by CD4 T cells. Because EAE stands as a representative such disorder, authors ask whether B cell-derived IL-6 could impact the development of Th17 population in the course of this disease. They demonstrate that it is indeed the case since the ablation of IL-6 expression in B cells is able to diminish the propagation of Th17 response during EAE. Remarkably, the B cell-derived IL-6/Th17 axis is operative regardless of antigen specificity as the Th17 population is also less numerous following immunizations with EAE-irrelevant OVA peptide. In the last part of paper devoted to human patients investigators show that the ability of B cells to control Th17 development through IL-6 release may be conserved across mammalian species.

The information that this report contains is obviously important because of its practical value but it also stimulates to ask broader questions. Why we are equipped with signaling pathways like the one described in the discussed report? To what end the apparently pathogenic (in the context of autoimmunity) B cell-derived IL-6/Th17 axis has evolved? Why B cells are so prone to act as autoimmunity mediators when stimulated with TLR ligands (especially nucleic acid-recognizing ligands)? The case of lupus and now that of multiple sclerosis have provided enough evidence for such ostensibly rebellious nature of B cells. Could potentially pathogenic pathways starting with the recognition of TLR ligands by B cells represent an evolutionary trade-off with the better control of gut commensal bacteria as an asset and the danger of autoimmunity as a liability? As a matter of fact MyD88 signaling in B cells has been shown to take part in accommodating the intestinal microbiota by preventing their systemic spread when colonic injury occurs (B Cell-Intrinsic MyD88 Signaling Prevents the Lethal Dissemination of Commensal Bacteria during Colonic Damage: Immunity. 2012; 36 (2): 228-38).

I have one more remark concerning the data that this paper presents. Authors show that CpG (TLR9 ligand) is more efficient that LPS (TLR4 ligand) in driving IL-6 secretion by B cells. Nucleic acid-recognizing TLRs are unusual because they are hidden from the cell surface to endolysosomal compartments. Apart from that, recent publications by Gregory Barton’s group reveal that TLR9, TLR7 and TLR3 (expressed by a macrophage cell line) require proteolytic processing prior to becoming functional detectors. Such feature is interpreted as an additional safety measure since ligands for these receptors are expressed both by host cells and pathogens. I am not really sure if it makes sense but nobody checked if the similar requirement for proteolytic cleavage exists in B cells.

Barr TA, Shen P, Brown S, Lampropoulou V, Roch T, Lawrie S, Fan B, O’Connor RA, Anderton SM, Bar-Or A, Fillatreau S, & Gray D (2012). B cell depletion therapy ameliorates autoimmune disease through ablation of IL-6-producing B cells. The Journal of experimental medicine, 209 (5), 1001-10 PMID: 22547654

The early microbiota species composition may influence the B cell development in children

We never walk alone; each of us is colonized by scores of bacterial species that live in our gastrointestinal tract. The colonization event occurs right after birth and has profound effects on our immune system – this sentence appears in every modern immunology textbook. There is, however, no such thing like one human microbiome as the species composition in our gut depends on many aspects. Diet, geographical region we live in or civilization level of society we belong to may influence to what species we are exposed and most probably dictate also many qualitative features of human immune responses. But we do know yet many technical details of the interactions between the human microbiome species arrangement and our immune system characteristics. I have found the publication that studies what impact on the development of B cell populations in the childhood may have the early microbiota species composition in the gut.

The link: http://www.jimmunol.org/content/188/9/4315.abstract

The work is performed on Swedish children and it contains two parts. In the first authors follow the development of B cells in the analyzed cohort of individuals along the period that ranges from right after birth to three years of age. B cells from peripheral blood (defined as CD20+ cells from the lymphocyte gate) are subdivided into CD5+ population (secreting mostly polyreactive IgM antibodies) and CD27+ subset (whose majority are class-switched memory cells). The first population forms the bulk of B cells in children, it expand soon after birth and gradually contracts to reach low level in adults. The second subset grows much slower; it attains its plateau at 18 months of life and continues at similar numbers at 36 months (although there is still much more CD5+ B cells than CD27+ B cells at that time). In the adult blood CD27+ population prevails over CD5+ B cells.

For the second part authors collect fecal samples in the same cohort of children during the early weeks of their life (1, 2, 4 and 8) and study the microbiota species composition in obtained samples. To detect bacterial groups present in the analyzed material they use combined microbiological (growth conditions), biochemical and molecular approaches that identify Escherichia coliEnterobacteria that are not E.coliStaphylococcus aureusBacterioidesEnterococci, BifidobacteriaLactobacilli and Clostridia. Next investigators attempt to correlate quantitative features of B cell development (counts of CD20+, CD5+ and CD27+ cells) along the infancy period with qualitative aspects of microbiota composition (presence or absence of given bacterial groups) during the early weeks of life. The major conclusion of this paper is that the early presence of species or bacterial groups like E.coli and Bifidobacteria in children guts associates with the increased numbers of CD27+ B cells at 4 and 18 months of their life. However, the colonization with S.aureus correlates inversely with CD27+ B cell counts at 4 month.

Obviously correlations do not necessarily indicate for the casual bond but I think the data presented in this report are interesting especially when you combine them with the information that authors provide in the discussion part. According to that E.coli and Bifidobacteria are thought to be classical infantile bacterial species but S.aureus is not. This organism is considered to be a skin bacterium; however, it colonizes the gastrointestinal tract of Swedish children with high penetration (~70%) and may slow down the development of effective immune responses. Such publications are important because the incremental appreciation of how microbiota species composition regulates human immunity may let us use this knowledge for practical purposes in the future.

Lundell AC, Björnsson V, Ljung A, Ceder M, Johansen S, Lindhagen G, Törnhage CJ, Adlerberth I, Wold AE, & Rudin A (2012). Infant B cell memory differentiation and early gut bacterial colonization. Journal of immunology (Baltimore, Md. : 1950), 188 (9), 4315-22 PMID: 22490441

Germinal centers in pathogenic SIV infection

The characteristic feature of HIV and some SIV infections is the chronic immune activation that probably drives the continuous CD4 T cell depletion and progression towards AIDS. What exactly causes this aberrant activation is not clear; however, traits inherent to the immune response seem to be more responsible than the virus itself. For example, there are data showing that the same strain of virus (SIV) can induce lasting but non-pathogenic infection in one primate species (sooty mangabeys) and devastating disease which resembles AIDS in the other (rhesus macaques). It may be therefore instructive to study details of immune response against SIV in pathogenic conditions. B lymphocytes are not spared from overall activation during HIV/SIV infection – in fact, sick individuals display hypergammaglobulinemia, or elevated levels of IgG antibodies. Switched antibodies are generally produced by cells that derive from the germinal center reaction where B lymphocytes interact with the unique subset of CD4 T cells called follicular helpers (TFH). Follicular helpers can be distinguished from other CD4 T cell subsets by their expression of CXCR5, ICOS, PD-1 and the transcription factor Bcl-6 as well as the high level of IL-21 cytokine secretion. TFH have been shown to be crucial for the successful generation of plasma cell and memory compartments. The recent paper takes a look at germinal centers during SIV infection in monkeys that develop AIDS-like disease.

The link: http://www.jimmunol.org/content/188/7/3247.abstract

Authors use immunohistological staining and flow cytometry to prove that PD-1 positive CD4 T cells (according to what we know TFH population) accumulate in germinal centers along disease progression in rhesus macaques infected with SIVmac239. They also correlate the enhanced PD-1 expression on CD4 T cells with remodeling of B cell populations that reside in lymph nodes (the trend is towards reducing naïve subset and enriching memory subsets) and the increased IgG secretion. Additionally, it is demonstrated that CD8 T cells are excluded from germinal centers (both in naïve and infected monkeys). Investigators conclude that the buildup of TFH population during pathogenic SIV infection may play the important role in shifting B cell activation status. Apart from that they suggest that germinal centers could form anatomical sites of potential viral escape due to the lack of CD8 T cells presence (and alleged insufficient control of infected CD4 T cells there).

The experimental setting of this publication is restricted to recording several immune parameters at time points that represent either acute or chronic SIV infection. However, it is interesting because it attempts the systematic documentation of changes in the immune response that may underlie the pathologic immune activation distinctive for some primate lentiviruses. The obvious question for the future is whether there may be any significant changes in germinal center reaction between species that develop pathogenic or non-pathogenic SIV infection.

Hong, J., Amancha, P., Rogers, K., Ansari, A., & Villinger, F. (2012). Spatial Alterations between CD4+ T Follicular Helper, B, and CD8+ T Cells during Simian Immunodeficiency Virus Infection: T/B Cell Homeostasis, Activation, and Potential Mechanism for Viral Escape The Journal of Immunology, 188 (7), 3247-3256 DOI: 10.4049/jimmunol.1103138

IgE class switching occurs in germinal centers

IgE antibody response is known to accompany infections with helminths. Organisms like Schistosoma mansoni, Nippostrongylus brasiliensis, Heligmosomoides polygyrus or Brugia malayi are able to induce the substantial class switching to IgE and increased levels of IgE serum antibody. IgE antibodies are also associated with allergies, asthma and generally Th2 effector responses. The other murine antibody class connected to Th2 is IgG1. Despite the importance of IgE response not all facts linked to basic IgE biology are entirely clear.  For example, it is not sure whether class-switching to IgE takes place in or outside germinal centers. Another elusive issue is the hypothetical presence of separate IgE memory population. There were some suggestions that IgE class switching needs to go through IgG1 intermediates and that IgE memory is maintained in IgG1 compartment. The recent publication uses IgE-reporter mouse strain to challenge the notion that IgE response occurs exclusively in tight connection with IgG1.

The link:
http://www.nature.com/ni/journal/v13/n4/abs/ni.2256.html

Authors use the fact that the secreted and membrane forms of antibody are translated from different mRNA splice variants. The IgE reporter strain they employ has the GFP sequence inserted downstream of exon that encodes the cytoplasmic domain of membrane IgE thus connecting the fluorescent signal with the membrane antibody version. Additionally, the IgE locus of reporter mice contains 52-amino acid region (called M1) derived from human antibody. Such approach allows detecting GFP signal as approximation of IgE membrane variant synthesis and subsequently verifying the presence of IgE with antibody against M1.

To confirm the validity of this strain for in vivo studies authors employ in vitro cultures with IgE response promoting factors (anti-CD40 and IL-4). It is established that GFP-positive population can be divided into GFPint and GFPhi subsets. The presence of soluble IgE correlates with appearance of GFPhi cells and only GFPhi fraction stains positively with anti-M1 antibody. Additionally, GFPhi cells express much more of IgE mature transcript than GFPint population which contains numerous cells positive for IgG1 presence. It is therefore concluded that only GFPhi fraction encloses IgE-switched cells.

To monitor how IgE-switched population comes into existence authors use the infection with Nippostrongylus brasiliensis. They aim to detect the germinal center (B220+IgDGL7+CD95+) and memory (B220+IgD+GL7CD38hi) compartments inside GFPhi (and thus IgE-switched) subset by flow cytometry. At the initial phase of infection all GFPhi cells bear features characteristic for germinal centers. At later stages some of them transform into memory cells. The appearance of IgE germinal center and memory populations occurs at the same time as analogous IgG1 subsets. Authors interpret these data as ruling against the possibility of IgG1 transitional stage for IgE-switched cells. Investigators also confirm the functionality of IgE memory population by adoptive transfer strategies. Additionally, the publication contains data about in vivo generation of IgE-secreting plasma cells and visualizations of germinal centers with IgE-switched cells (plus IgE memory and plasma cells) in lymph nodes of infected mice.

My first reaction after reading this paper was that of astonishment. Did they know so little about IgE response not to be sure whether there are IgE-switched cells inside germinal centers? The included data do not show anything very new or unexpected. They just extend basic facts from B cell biology for the IgE population.  Investigators show in the convincing way that IgE memory and plasma compartments derive from germinal center reaction and that memory IgE cells contribute to faster antibody response on re-infection. On the whole this is very interesting stuff which illustrates how many gaps are still in our knowledge.

Talay, O., Yan, D., Brightbill, H., Straney, E., Zhou, M., Ladi, E., Lee, W., Egen, J., Austin, C., Xu, M., & Wu, L. (2012). IgE+ memory B cells and plasma cells generated through a germinal-center pathway Nature Immunology, 13 (4), 396-404 DOI: 10.1038/ni.2256

MyD88 signaling in B cells prevents commensals from turning into pathogens

Several years ago Ruslan Medzhitov’s group has published the paper revealing that TLR signaling in response to commensal bacteria is important for the maintenance of intestinal homeostasis. In the model that uses chemical injury (DSS) to compromise the gut integrity such protectiveness provided by recognition of microbiota was completely counter-intuitive result. Why the massive inflammation did not occur when scores of bacterial derivatives have traversed from intestinal lumen to lamina propria and got exposed to the immune system? The recent paper analyses what cellular population is responsible for this unexpected phenomenon and delivers the equally unexpected result – they are B cells. Without MyD88 signaling in B cells commensals that inhabit the gastrointestinal tract show their bad face and turn into pathogens.

The link:
http://www.cell.com/immunity/abstract/S1074-7613(12)00014-3

Authors demonstrate that MyD88-deficient mice have compromised survival during DSS treatment and their increased mortality is dependent on the bacterial presence in the gut. The selective inactivation of MyD88 in various cell lineages (epithelial cells, dendritic cells, macrophages, T cells and B cells) shows that the observed mortality effect is due to MyD88 lack in B cells only. The B cell-specific MyD88 signaling seems to regulate IgM response to commensal bacteria. B cells do not need intrinsic MyD88 expression to secrete IgA, although when MyD88 is systemically deleted there is the significant reduction in IgA titers. Finally, the publication points out that IgM-driven complement deposition on microbiota could be accountable for MyD88-dependent homeostasis balance in the gut.

The adaptive response switched on innate-type signals that are received directly by B cells and in which absence commensal microbiota turn to pathogens – such data may have significance that extends beyond strictly practical implications (like new therapies). The intricate relationship between vertebrates and their bacterial symbionts and the impact of microbiota on host physiology are widely accepted facts. But how this association between members of different life domains did originate? I don’t have the answer, nobody probably has and most likely getting such answer is not possible without time machine. However, there are some interesting correlations to be made. Among animals vertebrates and agnathans stand apart as having both the adaptive immune system and the intestinal microflora (at least vertebrates, since I don’t remember reading any paper describing lamprey’s gut bacteria). And despite what an average immunologist trained on mouse models can think of invertebrates – they are highly successful and diverse animals, too. Consider just arthropods or mollusks – they can efficiently defend themselves without the adaptive immunity and spent much more time on this planet than we did. So, what was the evolutionary force behind the formation of adaptive immunity? Maybe it wasn’t protection against infectious organisms?

Kirkland D, Benson A, Mirpuri J, Pifer R, Hou B, Defranco AL, & Yarovinsky F (2012). B Cell-Intrinsic MyD88 Signaling Prevents the Lethal Dissemination of Commensal Bacteria during Colonic Damage. Immunity, 36 (2), 228-38 PMID: 22306056