Identification of an indirectly presented epitope in a mouse model of skin allograft rejection

The indirect pathway of allorecognition involves the processing and presentation of donor molecules by recipient antigen-presenting cells to alloreactive CD4+ T cells. Our objective was to assess the occurrence and significance of the indirect presentation of allogeneic major histocompatibility complex molecules in the rejection of major histocompatibility complex class I-disparate skin.

A mouse model of allograft rejection was developed in which tail skin from C57.BL/10 (H2b) donors was transplanted onto B10.A(5R) recipients resulting in an allogeneic mismatch at the D locus. T-cell depletion studies were used to characterize T-cell subset involvement in rejection. B10.A(5R) mice were immunized with pools of overlapping peptides spanning the polymorphic region of Db in order to identify Db-derived epitopes involved in rejection. The relevance of these epitopes was assessed through immunization of recipient mice with peptides before skin grafting to observe the effect of presensitization on the kinetics of rejection.

Rejection of Db-disparate skin by B10.A(5R) was delayed by CD4 and CD8+ T-cell depletion, indicating the significance of both cell types in rejection. At least six immunogenic peptides were identified, all of which contained a cryptic T-cell epitope. One peptide, however, was able to accelerate the rejection of Db-disparate skin. Presensitization of B10.A(5R) mice with this peptide also resulted in an increase in alloantibody, indicating the presence of a physiological as well as a cryptic epitope. Presensitization of mice with a peptide containing a distinct cryptic epitope, however, failed to influence rejection.

These findings demonstrate a significant role for the indirect pathway of antigen presentation in allograft rejection.