Targeting indirect pathway CD4 T-cell alloresponses in the prevention of chronic transplant rejection

Uniquely, alloantigen is recognised by two pathways: as intact antigen on the surface of donor antigen-presenting cells (direct) and as self-restricted processed allopeptide (indirect). The indirect pathway is believed to be longlasting, and is generally considered to be a single entity. Here we address how indirect responses against different alloantigens differ in their strength and longevity, and how this knowledge could be used to direct immunoregulatory therapy with antigen-specific regulatory T cells (Tregs).

A murine model of cardiac transplantation was used (bm12.Kd.IE to C57BL/6). Indirect CD4 T-cell allorecognition of mismatched donor MHC class I and II, and of H-Y minor histocompatibility antigen was assessed by quantifying proliferation of adoptively transferred monoclonal T-cell receptor transgenic T cells (TCR75, Tea, Mar). Antigen presentation by dendritic cells and B cells was assessed by selective depletion with diphtheria toxin or depleting anti-CD20 monoclonal antibody. Tregs were generated by in-vitro culture.

Indirect pathway responses were heterogeneous. Whereas the indirect response against class I alloantigen was longlasting and persistently strong, the response against class II alloantigen decayed within 2 weeks. Leucocyte depletion studies confirmed that this difference was due to rapid destruction of MHC class II expressing donor B cells and dendritic cells in the recipient, whereas anti-class I responses were generated by continual processing of graft parenchymal cells; recognition of donor haemopoietic fraction was not required. Notably, transfer of MHC class I specific Tregs at transplant or 3 weeks later abrogated germinal centre alloantibody responses and blocked development of allograft vasculopathy, whereas class II specific Tregs were ineffective when transferred at the late timepoint.

Although indirect allorecognition is considered to be a single entity, our results show that it consists of a number of responses that vary in duration and strength according to target alloantigen. The ability of class I allopeptide specific Tregs, but not class II specific Tregs, to prevent rejection when transferred at a late timepoint suggests that antigen-specific targeting of dominant and longlasting pathways might be particularly effective at preventing chronic rejection.

Wellcome Trust Clinical Research Training Fellowship.