Allopeptide-specific CD4⁺ T cells facilitate the differentiation of directly alloreactive graft-infiltrating CD8⁺ T cells

To investigate the mechanism of CD4⁺ T-cell help during the activation and differentiation of directly alloreactive CD8⁺ T cells, we examined the development of obliterative airways disease (OAD) following transplantation of airways into fully mismatched recipient mice deficient in CD4⁺ T cells. BALB/c trachea allografts became fibrosed significantly less frequently in B6 CD4^-/- recipients as compared to wildtype controls. Furthermore, class I-directed cytotoxicity failed to develop in the absence of CD4⁺ T cells. The infiltration of graft tissue by primed L^d-specific directly alloreactive 2C CD8⁺ T cells was not found to depend on the presence of CD4⁺ T cells. Nevertheless, graft-infiltrating 2C CD8⁺ T cells failed to express CD69 and granzyme B when CD4⁺ T-cell help was unavailable. Importantly, reconstitution of B6 CD4^-/- recipient mice with graft peptide-specific TCR-Tg CD4⁺ T cells (OT-II or TEa) capable of recognizing antigen only on recipient APC allowed for full expression of CD69 and granzyme B by the directly alloreactive CD8⁺ T cells and restored the capacity of recipients to reject their allografts. These results demonstrate that indirectly alloreactive CD4⁺ T cells ensure the optimal activation and differentiation of graft-infiltrating directly alloreactive CD8⁺ T cells independent of donor APC recognition.