Failure to tolerate gut microbiota transforms immunodeficiency into immunopathology

Here is a paradox for you that many don't even know that it exists: frequently, genetic immunodeficiency (weak immune response) syndromes are associated with immunopathologies (excessive immune response). So how could it be explained?   

New study published in Journal of Experimental Medicine may have some new answers. This study described mouse model of human immunodeficiency syndrome called, Omenn syndrome (after its discoverer) and showed that immunopathology was driven by gut microbiota.

Omenn syndrome is caused by hypomorphic (low active) RAG mutations. Analysis of intestinal tissue from Rag2^R229Q [Omenn] mice revealed pathological infiltration with inflammatory T cell subsets, T_H17 and T_H1.

Adoptive transfer showed that intestinal immunopathology was mediated by Rag2^R229Q mutant T cells.

Interestingly, Rag2^R229Q mouse harbored comparable numbers of Foxp3⁺ CD4⁺ T cells, implying that simple presence of regulatory T cells [generated in Rag2^R229Q mice] was not enough to control tissue inflammation.

Defect of Tregs derived from Rag2^R229Q mice was confirmed in adoptive transfer experiment with WT Tregs.

Besides T cell-driven immunopathology, Rag2^R229Q mice displayed IgA deficiency (failure to properly coat gut microflora). These data pointed to the possibility of microbial translocation causing excessive inflammatory response.

Indeed, antibiotic treatment of Rag2^R229Q mice could reduce intestinal immunopathology.

The role of antibiotic-sensitive gut flora in driving immunopathology in Rag2^R229Q mice was confirmed in adoptive fecal transfer experiments.

In summary, this study suggests the following scenario: hypomorphic RAG defect in Rag2^R229Q mice leads to "narrowing" of TCR and BCR repertoire. This in turn leads to outgrowth of oligoclonal T and B cells in Rag2^R229Q mice (wherein Rag2^R229Q mice contain T and B cells with limited, restricted, deficient TCR and BCR repertoires). Without proper TCR and BCR repertoire diversity, however, Rag2^R229Q mice fails to develop tolerance (IgA and Tregs) to gut flora or commensal microbial antigens present at mucosal surfaces (such as lung, intestine). Repertoire restriction also leads to failure to mount adequate and proper immune response.

David Usharauli

Deceptive strength applies to T cells as well

This week journal Nature Immunology published interesting study examining the correlation between TCR affinity and T cell competence. Using M. tuberculosis infection model, the authors showed that strength of antigen recognition by TCR reduces T cell competence long-term.

For this study the authors compared control of M. tuberculosis infection by two T cell receptor (TCR) transgenic mouse, named C24 and C7. In C24, CD4+ T cells express a TCR with high avidity [affinity, here] for M. tuberculosis antigen ESAT6, while in C7, CD4+ T cells express a TCR with intermediate avidity for ESAT6. Surprisingly, adoptive transfer of in vitro generated T_H1 cells from high avidity C24 mice provided significantly less protection against lung M. tuberculosis infection compared to Th1 cells from intermediate avidity C7 mice.

Interestingly, C24 Th1 cells showed more significant down-regulation of surface TCR post-transfer in both infected and un-infected hosts [it appears that in vitro activated Th1 continue to proliferate independent of antigen].

In addition, while TCR down-regulation was a general feature of activated CD4+ T cells (including endogenous, non-transgenic ESAT6-specific T cells), such TCR down-regulation was profound and long-term for C24 T cells.

Importantly, unlike C7, C24 Th1 cells transferred to uninfected hosts and harvested at day 13 showed drastic reduction of antigen-specific cytokine production.

Moreover, mice transferred with naive C24 T cells and infected with L. monocytogenes–ESAT6 and reinfected 1 month later showed failure to mount secondary response.

In summary, this study revealed that high avidity (affinity) T cells do not necessarily represent the best T cells to control infection. We could extrapolate this observation to include T cells with high affinity TCR against tumor-specific antigens. Important consideration in designing TCR transduced T cells for adoptive T cell immunotherapy.

David Usharauli

Melanoma patients harbor tumor mutation-specific PD1+ CD8 T cells in the peripheral blood

Steven Rosenberg's research group at National Institutes of Health (NIH) continues to define and refine condition for T cell-based cancer-specific immunotherapy. (for example, introduction of high-throughput personalized screening strategy capable of evaluating T cell reactivity to neo-antigens presented on all of the HLA restriction elements of the individual).

This time, their group showed that peripheral blood of melanoma patients harbors small but distinct PD1+ CD8 T cell population enriched in specificities for cancer associated neo-antigens.

If one compares CD8 T cells from PBMC v Tumor sites (TIL), blood derived T cells contains few PD1⁺/PD1^high CD8 T cells.

However, when the authors has expanded in vitro those sorted PD1⁺ CD8 T cells and co-cultured them with autologous dendritic cells expressing tumor neo-antigens (as tandem minigenes, TMG), they could identify circulating neoantigen-reactive CD8 T cells in three of the four melanoma patients evaluated.

Then the authors re-constructed blood PD1⁺/PD1^high CD8 T cell TCR specificity by (a) pairing the sequences encoding the two most-dominant TCR-α and TCR-β sequences, (b) cloning them into retroviral vectors and (c) transducing autologous PBMC. This TCR construct could [for example] detect neo-antigens derived from mutations in the genes MAGE family member A6 (MAGEA6).

Importantly, both PD1⁺/PD1^high CD8 T cells enriched from peripheral blood or T lymphocytes transduced with retroviruses expressing neo-antigen-specific TCRs could detect autologous tumor cell lines.

Finally, the authors found that blood and tumor site derived PD1⁺ CD8 T cell showed high degree of overlap in their TCR specificity [to tumor neo-antigens], suggesting that analysis of peripheral PD1⁺ CD8 T cells from cancer patients could reveal TCR specificities of tumor infiltrated lymphocytes.

In summary, this study is another evidence that cancer immunotherapy holds great promise in providing cancer antigen-tailored treatments. Identification of cancer neo-antigen specific T cells (TCRs) as shown in this study, would accelerate development of tumor-specific TCR constructs and could contribute in overcoming precursor limitation inherent to endogenous T cell clones.

David Usharauli