Immunisation with an allogeneic peptide promotes the induction of antigen-specific MHC II(pos) CD4+ rat T cells demonstrating immunostimulatory properties.

Background: The phenomenon of T cell stimulation by MHC class II expressing (MHC II(pos)) CD4+ T cells has been intensively investigated for T cell clones but, so far, not for native T cells. The extensive use of T cell clones may explain the inconsistent outcomes of T cell-mediated antigen-presentation. Therefore, we used freshly isolated primed rat CD4+ T cells induced by immunisation with an allogeneic peptide P1, which is involved in allograft rejection.

Methods: MHC II(pos) and MHC II(neg) CD4+ T cells were isolated from popliteal lymph nodes of P1-immunised Lewis rats and were purified by combining depletion and positive selection steps. Purified MHC II(pos) CD4+ T cells and MHC II(neg) CD4+ T cells (10⁵ cells per well each) were autostimulated or restimulated with P1-loaded (33 μg/ml peptide P1) and subsequently irradiated (with 20 Gy) autologous DC.

Results: Seven days after immunisation, a small population of MHC II(pos) CD4+ T cells was detectable (approximately 8.0% of total lymph node cells), as well as a large population of MHC II(neg) CD4+ T cells (up to 45%). Antigen-specific proliferation was observed for both T cell populations but only P1-loaded MHC II(pos) CD4+ T cells presented antigen presenting cell (APC) function for P1-primed T cells. Their inability to activate unprimed T cells may be due to impaired surface expression of costimulatory molecules (CD80 and CD86).

Conclusion: Immunisation with the allogeneic peptide antigen P1 induced antigen-specific MHC II(pos) CD4+ rat T cells demonstrating perfect APC function for primed T cells in vitro.