DNA associated with the cell membrane is involved in the inhibition of the skin rejection response induced by infusions of photodamaged alloreactive cells that mediate rejection of skin allograft.

Cell membrane DNA (cmDNA) is a form of DNA located on the surface of human and murine T-cells. It has recently been characterized as a target for photomodification by 8-methoxypsoralen (8-MOP) and long-wave ultraviolet light (UV-A). Whereas 8-MOP itself is biologically inert, photoactivated 8-MOP is covalently bound to pyrimidine bases in DNA. We have investigated the possible involvement of cmDNA photomodification in the induction of the suppression of skin allograft rejection in BALB/c mice preimmunized with 8-MOP/UV-A photodamaged alloreactive cells which mediates this allograft rejection. This suppression is demonstrated by inhibition of delayed-type hypersensitivity (DTH) and mixed leukocyte culture (MLC) responses. Splenocytes from BALB/c mice undergoing rejection of CBA/j skin graft which contained an expanded population of the effector T lymphocytes that mediate the rejection were treated with DNAse to remove cmDNA before or after treatment with 8-MOP and UV-A prior to infusion into naive BALB/c recipients. Mice that received pretreated effector cells were tested for MLC responses to CBA/j or B10 alloantigens before and after the DTH response. The DTH response of all groups of pretreated BALB/c mice to the relevant alloantigen was specifically suppressed as compared with the response of control mice. However, adoptive transfer of the suppression of the DTH response was optimally demonstrable only in syngeneic recipients of cells from donor mice treated with photodamaged alloreactive cells. Also, splenocytes from BALB/c mice immunized with photodamaged alloreactive cells demonstrated highly significant hyporesponsiveness and suppression of the MLC response of naive mice to the relevant alloantigen in the case of the primary MLC response, and to both alloantigens in the secondary MLC response which was totally eliminated by prior pretreatment of these effector cells with DNAse. Therefore, it appears that the suppression of the DTH response can be induced by pretreatment of the effector cells with DNAse and/or 8-MOP and UV-A but is adoptively transferable optimally only from mice which are recipients of photodamaged alloreactive cells. Moreover, the effectiveness of this treatment is decreased by prior removal of cmDNA from these cells. The presence of cmDNA is necessary for induction of suppression of the primary and secondary MLC responses in mice treated with photodamaged cells of allograft rejection.