Introduction

We have previously described a partially-mismatched [bm12.Kd.IE (IA^bm12, IE^d, K^d, K^b, D^b) to C57BL/6] model of chronic cardiac allograft rejection, and demonstrated surprisingly that indirect CD4 T cell allorecognition of donor MHC class II alloantigen occurred only transiently. This work aims to identify the factors responsible for limiting the duration of this response.

Methods

Indirect CD4 T cell allorecognition of donor MHC class I and II was assessed by quantifying proliferation of, respectively, CFSE-labelled TCR-transgenic TCR75 (H2-K^d peptide-specific) and TEa (I-E peptide-specific) CD4 T cells, adoptively transferred on day 0 or 28 following transplantation of bm12.Kd.IE heart allografts into either wild-type or RAG2KO C57BL/6 recipients. The role of donor dendritic cells in priming indirect allorecognition was assessed by incorporating BALB/c.DTR donors, enabling ablation of donor DCs by diphtheria toxin treatment, and by adoptive transfer of cultured bone-marrow derived dendritic cells (BMDC’s).

Results

Whereas extensive proliferation of TCR75 CD4 T cells was observed in heart-grafted recipients at both early and late time points, TEa CD4 T cell proliferation was detectable only immediately after transplant. We hypothesised that the early termination of the anti-class II response reflected rapid loss of donor DCs; in support, donor DC depletion at transplantation resulted in attenuation of the indirect MHC class II response but only a modest reduction in class I responses. Donor DCs were not killed by NK cells, because Bm12.Kd.IE cells survive long-term in NK-cell replete, RAG2KO recipients. Equally, late anti-class II indirect responses in RAG2KO heart-grafted recipients were detectable, albeit weaker than the response immediately following transplantation, suggesting that donor DCs are instead lost to natural senescence and adaptive immune killing. Critically, transfer of donor BMDC’s into RAG2KO recipients 28 days after heart grafting completely restored late TEa responses, but did not restore TEa proliferation in WT recipients, presumably because rapid killing of donor DC by primed adaptive alloimmunity prevents antigen presentation.

Conclusion

The longevity of indirect-pathway CD4 T cell responses vary according to target antigen. The anti-class II response terminates early, likely because donors DCs, the major source of delivery of MHC II alloantigen, are depleted rapidly.

« Back to 2013 American Transplant Congress