Acceptance of class II major histocompatibility complex disparate skin grafts associated with suppressor cells and elevated Langerhans cell numbers.

Class II major histocompatibility complex (MHC) molecules are only present on Langerhans cells (LC) in normal murine epidermis. Depletion of this antigen with the chemical carcinogen dimethylbenzanthracene (DMBA) causes I-E disparate B10.A(2R) congenic tail skin to be accepted permanently when grafted onto B10.A(4R) recipients. Adoptive transfer of spleen cells from these recipients into naive syngeneic hosts inhibited the ability of the host mice to reject untreated B10.A(2R) tail skin grafts. Hence DMBA-treated LC depleted I-E disparate skin grafts activate suppressor cells which did not inhibit BALB/c mice from rejecting a B10.A(2R) tail skin graft. In contrast, the tobacco derived carcinogen benzo(a)pyrene (BP) increased the number of epidermal LC but had no effect on either class I or class II MHC disparate skin graft survival time. This confirms that the number of class II MHC-positive LC is critical for the initiation of skin graft rejection; when the threshold level is attained graft rejection proceeds at a maximal rate that cannot be enhanced by raising the number of LC. The tolerant skin grafts had increased numbers of LC; this was not observed in syngeneic grafts and therefore may be related to the active suppression of immunity.