Heterogeneity in Indirect Pathway CD4 T Cell Alloresponses.: Abstract# B766

Background The indirect pathway is believed to be long-lived, responsible for chronic rejection, and thought of as a single entity. Here we address how indirect responses against different alloantigens differ in their strength and longevity, and how this knowledge may be utilised with therapeutic administration of antigen-specific regulatory T cells (Treg). Methods A murine model of cardiac transplantation was utilised [bm12.Kd.IE to C57BL6]. Indirect CD4 T-cell allorecognition of donor MHC class I and II, and H-Y minor antigen was assessed by quantifying proliferation of adoptively transferred monoclonal T-cell receptor transgenic T-cells (TCR75, TEa and Mar respectively) at various time points. Antigen presentation by dendritic cells (DC) was assessed by selective depletion using diphtheria toxin; and B-cells, by administering depleting anti-CD20 monoclonal antibody. Treg were generated by in vitro culture of CD4 T cells. Results Indirect pathway responses were heterogeneous. Whereas the indirect response against class I alloantigen was long-lived and persistently strong, the class II indirect response was remarkably short-lived (decaying within two weeks), because it is dependent upon donor B-cells and DC's as a source of class II alloantigen, and these are cleared rapidly by the recipient. The longevity of the class I indirect response reflected on-going antigen presentation, but notably B-cells played an increasingly important role, perhaps reflecting antigen-specific expansion. The indirect response against minor H-Y antigen was long-lived but weakened progressively. Finally, in keeping with identification that the class I indirect response is the dominant pathway long term in this model, transfer of class I antigen specific Treg (TCR75) either at the time of transplant, or three weeks later, resulted in the abrogation of germinal centre responses and the development of allograft vasculopathy. Discussion Although thought of as a single entity, our results highlight that indirect allorecognition comprises a number of responses that vary in duration and strength according to target alloantigen. Targeting those responses that are long-lived, for example, with Treg, may be particularly effective at preventing chronic rejection.