The hidden nature of TCR

The adaptive immune system uses the anticipatory receptor strategy as a mean to detect foreign antigens. Despite that these anticipatory receptors are arranged by similar gene recombination mechanisms in two main lymphocyte types they are not alike in the way they operate. Receptors on T cells are MHC restricted which means that they can recognize exclusively pre-processed linear peptides bound to major histocompatibility complexes. In contrast receptors on B lymphocytes bind to antigens that remain in their native conformation. The recent publication providing data on the very basics of MHC restriction can teach us more about the hidden nature of TCR.

The link:

The report aims to distinguish whether MHC restriction stands as an intrinsic feature of TCR or could be imposed on T cells during their development. The molecular switch that underlies MHC restriction consists of TCR, CD4/CD8 co-receptors and Lck kinase. The role of co-receptors is to enforce that signaling from TCR occurs exclusively in the context of MHC binding. Co-receptors engage Lck kinase and make it available to TCR only when they are attached to MHC. Immature T cells start expressing CD4 and CD8 during their compulsory development stage in the thymus.

To make the distinction possible authors analyze binding specificities of TCRs that underwent the thymic selection in the joint absence of MHC and CD4/CD8 co-receptors. If the restriction feature was built-in to TCR structure than the sheer absence of MHC would yield TCRs unresponsive to any ligand. However, the lack of MHC and CD4/CD8 may as well allow the positive selection of receptors that recognize unprocessed antigens because the availability of Lck kinase to TCR would be independent of MHC binding. In such circumstances MHC restriction could be interpreted as enforced by the thymic selection. Authors show that this is indeed the case. Data indicate that TCRs educated without MHC and co-receptors bear the striking similarity to B cell receptors. They display affinity for antigens in the native conformation as well as bind their ligands as soluble proteins.

The main conclusion from this report has enticed me to do some musing about the evolutionary origin of lymphocyte. Obviously, vertebrate adaptive immune compartments with all their diversity, flexibility and mutual dependability did not start as we see them today – they had to be less complicated in the past. Could T cells originate from a B cell? I know that I’m looking like a fan of disproven recapitulation theory (which I’m not), but B cells seem like intuitive candidates for a primeval lymphocyte. They recognize antigen in the simpler way than T cells do – without the need for external support in the form of antigen presenting cell. But I am not entirely sure if you can extend these data so far. Probably you can’t.

The link between Th17 and HIV pathogenesis

The Th17 population is a CD4 T lymphocyte effector subset that in the simplest way can be defined as IL-17A releasing cells; although several other cytokines and transcription factors seem to be typical as well for cells that express IL-17A. These cells are protective in several bacterial and fungal diseases, particularly at mucosal parts of our body. They also are regarded as pathogenic during autoimmune conditions and the infection with Schistosoma mansoni parasite. Another interesting feature of Th17 population is the link to HIV pathogenesis. HIV virus replicates within Th17 subset and IL-17A expressing cells are preferentially depleted from the gut mucosa during HIV infection. The reduction in Th17 level has been linked to increased permeability of mucosal barrier in the digestive tract. This can lead to continuous leaking of microbial products into the bloodstream and maintains chronic immune activation – the HIV infection distinctive feature. Two recent publications reinforce the connection between Th17 and HIV.

The first link:

This publication presents data collected in the region that is affected by AIDS pandemic. Subjects of the study are Kenyan women divided into two pools – virus-negative healthy controls and HIV-positive sex workers. HIV tends to infect activated CD4 T cells that express certain co-receptors facilitating the virus entry. Additionally, α4β7 integrin known to be involved in the mucosal homing binds HIV gp120 envelope protein. Accordingly, authors perform the comparative analysis of cervical and blood CD4 T lymphocytes from healthy donors. This analysis includes enumeration of mucosal homing markers (CCR9, α4β7 and CD103), the early activation marker CD69 and HIV co-receptors (CCR5 and CXCR4).

The research aims to estimate features of potential invasion targets in the female genital tract. As the first conclusion authors have found that cervical CD4 T lymphocytes are enriched for several indicators of HIV susceptibility with α4β7 integrin, CD69 and CCR5 often being co-expressed on individual cells. It has been also established that cervical CD4 T lymphocytes include cells positive for IL-17A. The majority of these IL-17A-positive cells express α4β7 integrin and CCR5 which makes them probable targets during the initial infection stage. Next, authors examine IL-17A expression levels in blood and cervical samples from infected women. The study concludes with the evidence that IL-17A-positive CD4 T cells are strikingly depleted from the cervix mucosa of diseased participants but still present in their blood.

The second link:

Data reported in this article have been obtained from European healthy donors and patients infected with HIV that are treated with anti-retroviral therapy. People infected with HIV are continuously loosing their CD4 T cells and as a result develop the permanent immune deficiency. The anti-retroviral therapy helps rebuilding overall level of CD4 T cells but often proves ineffective in the restoration of intestinal CD4 T cell count. Investigating this matter authors focus on CD4 T lymphocytes from the small intestine and the blood to compare their gut-homing markers expression (α4β7 integrin and CCR9). Healthy controls harbor the vast majority of double positive β7+CCR9+ cells in the gut. However, infected subjects demonstrate the conspicuous reversal in proportions with most of β7+CCR9+ population being present in the blood. Additionally, HIV-positive individuals display the reduction of CCL25 (CCR9 ligand) expression level on intestinal epithelial cells.

This result implies that HIV infection may disturb the redistribution of CD4 T lymphocytes to the gut by unbalancing the CCR9-CCL25 interaction. Authors also associate the low level of intestinal β7+CCR9+ CD4 T cells with increased indicators of damage to the intestinal epithelium. Finally, it is demonstrated that β7+CCR9+ subset is richer in IL-17A secreting cells than β7+CCR9 population. However, in most of their experiments authors did not discriminate between Th17 and other CD4 T cell subsets.

I am following the Th17-HIV story because it emphasizes certain point, which I think is important from the immunologist perspective. Data seem to indicate that HIV infection drives the reduction of Th17 cells at mucosal surfaces. This reduction seems to be irreversible and it disturbs the intestinal homeostasis. Changes in mucosal environment may lead to the enhanced rate of microbial translocation which fuels the unique HIV trait – systemic immune activation. The link between Th17 and HIV pathogenesis makes me wonder whether the consequences of Th17 depletion from mucosal sites arise because of insufficient protection versus enteric pathogens or inadequate accommodation of intestinal commensals.

Our immune system is often metaphorically viewed as well trained and properly equipped coercive body that provides protection against foreign invaders. That’s right – the immune system does provide protection and it is very good at it. But such metaphor might be too narrow to adequately reflect what our immune system actually is, especially at mucosal surfaces. After all we are inhabited by diverse commensal organisms to be reckoned with but not to be disposed of.

Obviously our digestive tract needs protection as it is constantly exposed to harmful bacteria and sometimes the distinction between pathogen and commensal is not entirely clear. Additionally, pathogens are equipped with virulence tools like secretion systems that may provide them with possibilities to stand out from the crowd. But if you consider the sheer biomass argument, commensal organisms prevail over pathogens by such enormous margin that their proper accommodation definitely looks like more urgent matter than the efficient protection against few marauders. So returning to the insufficient protection versus inadequate accommodation dilemma – my gut feeling is that the second option might be more correct.

Neutrophils and marginal zone B cells – a new partnership

Being interconnected – that’s one of cardinal features pertinent to the mammalian immune system. Cellular subsets involved in an immune response seldom act alone beyond the initial signal recognition stage. Instead, the converging and augmentation of warning signs, the induction, adjustments or limitations of effector branches and finally the maintenance of immunological memory – these steps are usually carried out by two or more interacting populations. It is always so interesting to learn about new connections between cells of immune system, therefore recent report describing the supportive role of neutrophils in the antibody secretion process has immediately caught my attention.

The link to the publication:

This article reminds me the other, now pretty notorious story about the partnership of germinal center B cells and follicular helper CD4 T cells – an interaction that’s been proven to be so important during the antibody maturation process. But we have quite other players this time around.  Marginal zone B cells form the population posed at the anatomical intersection between circulation and lymphoid areas in the spleen. They are considered to possess blood filtering functions and are critically important in sepsis. There also were reports describing their pre-activated phenotype, which means that they can be easily stimulated to make antibodies. Neutrophils are most numerous granulocytes; they are cells extremely proficient in phagocytosis – kind of the always-ready emergency crew generally thought to be the first line of defense.

The research is performed on human cells combining imaging and functional data. The paper starts with series of pictures showing that neutrophils can colonize the splenic marginal zone under homeostatic but not inflammatory conditions. As a control authors perform the identical staining procedure with secondary lymphoid organs that have no marginal zone like tonsils, Peyer’s patches and peripheral lymph nodes. Then, compelling and diverse evidences are presented on how neutrophils enhance repertoire diversification and antibody secretion by marginal zone B cells. Apart from that, authors provide an in-depth description of the novel neutrophil “B helper” phenotype that is attained on contact with splenic epithelial cells.

However, it is the last and shortest paragraph of result section that really draws my attention. According to authors the increase in splenic colonization by neutrophils correlates with postnatal microbial colonization and fluorescence microscopy has shown LPS presence in adult but not fetal spleens. LPS was also shown to boost reprogramming of neutrophils to the “B helper” phenotype by splenic epithelial cells. Additionally, germ-free mice displayed lower levels of “B helper” neutrophils. Authors state – “Thus, we propose that TLR signals from mucosal commensals enhanced the splenic recruitment and reprogramming of N BH cells to enhance innate MZ B cell responses to highly conserved microbial TI antigens”.

Postnatal microbial colonization is the process of inhabiting intestines by commensal bacteria known as microbiota. It occurs soon after birth and has the instructive as well as regulatory effect on our immune system. Obviously the innate arm of immunity has the ability to detect microbial products and convey signals to start the inflammation. However, under normal circumstances the presence of microbiota is accommodated, although how exactly it happens is not entirely clear. Before I’ve read this paper I thought that so called microbial translocation or in other words the leakage of bacterial material from intestines into the bloodstream could happen exclusively as a pathological process.  Our body needs to be kept sterile; otherwise fatal disaster like sepsis may develop. So how to explain LPS presence in the spleen detected under homeostatic, non-inflammatory conditions? The most intuitive answer would be that the low-level trickling of unwanted bacterial stuff from the gut is taking place on the normal basis. Hence neutrophils may be assigned the duty of “garbage workers”, especially around big lymphoid organs like spleen.

I wish that somebody confirmed the neutrophils/marginal zone B cells partnership in murine models that may allow manipulation with relevant genetic backgrounds and cellular populations. The obvious problem to be solved is what will happen when there are no “B helper” neutrophils around. Will that impair the ability of marginal zone B cells to mount the effective protection against blood-borne antigens? However, it is not the only question I would ask. Authors present data showing that “B helper” neutrophils are able to contain CD4 T cells’ proliferation, although they never really explore this feature. May their function be more inhibitory than stimulatory? Somehow, that unexpected LPS presence in spleen keeps on intriguing me.  But now I have no idea how to interpret that.