Intestinal Immune Conditioning with Helminths Employs Host T Helper 2 (Th2) Pathway to Induce Mixed Chimerism and Regulate Graft-Versus-Host Disease

Background

Achieving mixed chimerism suppresses donor T cell alloreactivity and reduces graft-versus-host disease (GVHD)-related mortality after bone marrow transplantation (BMT). Mixed hematopoietic chimerism is the only condition proven to lead to immune tolerance after transplantation. It can be achieved with protocols utilizing chemotherapeutic agents or strategies that block T cell activation or deplete host T cells. Selective irradiation of the thymus or lymphoid organs can also lead to mixed hematopoietic chimerism. After total lymphocyte irradiation (TLI), mixed chimerism is mediated by host iNKT lymphocytes and the T helper 2 (Th2) cytokine IL4 generated by those cells. IL4 signals through IL4 receptor (IL4R) and STAT6. After a toxic myeloablative pretransplant regimen, total body irradiation (TBI), intestinal immune conditioning with helminthic commensals regulates the host immune system and promotes a transient mixed chimerism. Conditioning with helminths also induces IL4 production by host cells and regulates GVHD while preserving the graft-versus-tumor effect. We hypothesize that induction of host Th2 pathway is critical for achieving mixed chimerism by intestinal immune conditioning

Methods

We evaluated the role of recipient Th2 signaling in the establishment of mixed chimerism in an MHC I/II major mismatch (H2b→H2d) model that utilizes TBI. Three weeks after infection with the mouse nematode, Heligmosomoides polygyrus-bakeri (Hpb), GVHD was induced by the delivery of T cell-depleted bone marrow (TCD-BM) and splenic T cells from uninfected WT C57BL/6 (MHC:H2b) donors into lethally-irradiated WT BALB/c, interleukin 4 (IL4) -/-, IL4Rα-/- and STAT6-/- recipients (MHC: H2d). Cellular composition for donor vs recipient cells, GVHD-mediated inflammation in end-organs (colon and lung), and survival of mice in each setting were assessed

Results

In 3 models of disruption of the Th2 pathway, we demonstrated that Th2 signaling in the host is essential for the establishment of mixed chimerism. Helminth infection promotes mixed chimerism in WT BALB/c BMT recipients; recipient T cells constitute 3-5% of peripheral population at 6 days post-BMT, whereas recipient T cells constitute <1-2% of population in uninfected BMT recipients (p<0.001). By contrast, when recipient cell IL4, IL4Rα or STAT6 is silenced, the percentage of recipient T cells remains negligible (<1% of total T cell population) in uninfected or Hpb-infected BMT recipients (p: NS between uninfected and Hpb-infected) (fig 1). Hpb infection promotes chimerism in mesenteric lymph nodes (MLN) of iNKT-deficient (Jα18-/-; MHC: H2d) BMT recipients of C57BL/6 donors (p<0.05 between uninfected and Hpb-infected Jα18-/- BMT recipients). Although helminths regulate GVHD-related inflammation in lungs and the colon of WT BALB/c recipients (p<0.05 between uninfected and Hpb-infected WT BALB/c BMT recipients), they fail to do so in these organs in IL4-, STAT6- or IL4Rα-deficient BMT recipients (p: NS between uninfected and Hpb-infected BMT recipients for each Th2 gene deficient group)(fig 2). Moreover, helminth infection did not promote survival of Th2-/- BMT recipients, where all mice died of lethal GVHD, although helminth infection promotes survival in WT BALB/c BMT recipients (P<0.05 between Hpb-infected WT BMT recipients and each other group). Because tolerance of alloreactive cells in mixed chimerism occurs with the induction of CTLA4 on T cells (a check point inhibitor and critical immune regulatory protein in GVHD), we also explored CTLA4 expression on peripheral T cells and found that helminth-induced CTLA4 production on splenic T cells was dependent on expression of IL4 and STAT6. In IL4-/- or STAT6-/- mice, there was no increase in the percentage of CTLA4+ T cells after Hpb infection (p: NS between uninfected and Hpb-infected BMT recipients for each Th2 gene deficient group), whereas helminth infection increased 3-fold the percentage of CTLA4+ T cells (p<0.05 between uninfected and Hpb-infected WT BALB/c mice)

Conclusions

Intestinal immune conditioning with helminths promotes mixed chimerism after toxic, host cell depleting myeloablative TBI and regulates lethal GVHD following BMT. Host cell Th2 pathway is critical for achieving mixed chimerism. Helminths stimulate CTLA4 - a cellular regulatory protein known to promote mixed chimerism after BMT- in a Th2 dependent manner

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