Current Research and Scholarly Interests
Our recent studies have challenged the paradigm that NK cells are not active participants in outcomes after solid organ transplantation.. We have demonstrated that recipient-derived NK cells not only infiltrate rejecting liver grafts early after transplantation but also are a significant source of IFNg thus bridging the innate and adaptive immune responses post transplant. In a high responder, DA->Lewis rat orthotopic liver transplant model we demonstrated that depletion of NK cells markedly prolongs graft survival in half of graft recipients and that there is a concomitant increase in expression of NKG2D ligands on hepatocytes (Zhuo et al, 2010). Interestingly, transplantation in the converse direction (Lewis->DA) is a model of spontaneous liver tolerance and we have shown that depletion of NK cells prevents the establishment of tolerance but once tolerance is established, NK cell depletion has no effect. Our studies support a role for NK cells in both graft rejection and tolerance.
NK cells express a wide range of receptors that have activating or stimulatory functions. We examined the activation receptors involved in the interaction of NK cells with dendritic cells (DC) since NK cells have the potential to mediate both maturation and killing of DC and this could alter T cell priming post-transplant. We used RNAi techniques to knock down the activation receptors, NKp46, NKp30 and NKG2D and demonstrated that DC stimulation of NK cells to produce IFNg; is mediated through NKp46 (Wai et al, 2010). NK cell killing of DC is independent of NKp30, NKp46 and NKG2D. We further examined the interactions of NK cells with tumors and determined that NK cell stimulation by tumor requires NKp30 while NK cell cytotoxicity is mediated by both NKp30 and NKG2D. Our studies indicate that NK cells interact with DC and tumors through distinct activating receptors – NKp46 and NKp30/NKG2D respectively. These data are important for the development of therapeutics that will target NK-DC interactions post-transplant and support the feasibility of this approach without compromising the ability of NK cells to kill virally-infected or transformed cells.Ongoing studies, are examining the functional role of NK cell receptors using many state-of-the-art techniques (microscopy, cyTOF, experimental systems).
Emerging studies, including results from our lab, implicate miRNAs in the pathogenesis of disease and recent reports describe the expression of miRNAs in transplant recipients. The functional significance of miRNAs in alloimmune responses and transplantation has not yet been elucidated. Recently we have reported that miRNAs are differentially expressed after transplantation and that miR-182 was significantly increased in rejecting cardiac allografts and in the mononuclear cells that infiltrate the grafts. We further showed that miR-182 targets foxo1 mRNA, a member of the Forkhead box (FOX) protein family of important transcription factors. As miR-182 increases after transplant, there is a concomitant post-transcriptional decrease in FOXO1 protein expression in heart allografts that is localized to both the cardiomyocytes and CD3+ T cells. These studies are the first to demonstrate a mechanistic role for a miRNA in transplantation and thus we are uniquely positioned to move the field forward in our understanding of how miRNAs contribute to alloimmune activation and regulation.
As part of a new large consortium grant (awarded to our group at Stanford), we will be analyzing plasma microRNAs from pediatric recipients of solid organ allografts to develop a non-invasive biomarker for post-transplant lymphoproliferative disorder (PTLD, a serious EBV associated compliaction of transplant). Three inter-related objectives are proposed: 1) to determine a profile of microRNAs that is diagnostic of PTLD, 2) to determine if microRNA expression correlates with specific LMP1 variants, and 3) to determine if microRNAs are predictive of PTLD.