NK cells and transplantation

The requirement for cytotoxic T lymphocytes during allograft rejection is controversial. We have demonstrated that CD8⁺ T cells are not essential for allograft rejection or for the induction of apoptosis in two experimental models of transplantation. To determine candidate cells types which may play a role in the events leading to graft rejection, the cellular composition of rejecting allografts was determined. We demonstrate that substantial numbers of NK cells, of recipient origin, infiltrate allografts as early as 12 h after transplantation. These NK cells produce cytokines and express cytotoxic mediators such as granzyme B and FasL. It is unknown which NK cell receptors are expressed and activated during transplantation. NK cells express multiple cell surface receptors, including MHC class I binding inhibitory receptors, which deliver a negative signal, and activation receptors, which stimulate cytokine secretion and cytotoxicity of NK cells. To begin to understand NK cell activation in the context of transplantation, we have recently cloned a novel rat immunoglobulin-like surface receptor from a rejecting liver allograft. Sequence analysis demonstrates that this putative activation receptor contains 71% identity to human NKp30 at the DNA level, suggesting that it is the rat homologue (rNKp30). Characterization of NK activation receptors may lead to better understanding of the interactions between the innate and adaptive immune responses in transplantation.