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