John A. Hansen

Biology of the Human MHC and Transplantation Tolerance

The human major histocompatibility complex (MHC), or HLA system, comprises multiple linked genes that play critical roles in regulating cellular immune responses and immunity. T cells recognize antigen derived peptides that are selected and presented by HLA class I and class II molecules. Natural killer (NK) cells and some T cells have inhibitory and stimulatory receptors that recognize epitopes encoded by HLA class I and class I-like molecules. Genetic variation in HLA genes is associated with susceptibility and resistance to infection, autoimmune disease and cancer. Genetic variation in HLA genes and differences between donors and recipients are major barriers to successful transplantation. Hematopoietic stem cell transplants (HSCT) from normal donors, familial or unrelated, may be rejected or graft-versus-host disease (GVHD) may result secondary to alloimmune reactions induced by HLA disparity.

Current studies in this lab are aimed at understanding the relevance and minimal requirements for genetic matching of donor and recipient in HSCT, the utility of genetic testing and functional assays to guide donor selection, and the mechanism by which tolerance occurs following HSCT. Studies are also underway to identify functional genetic variation in cytokine and other immune regulation genes in donors and recipients, and to determine if these genomic differences contribute to regimen-related toxicity, GVHD, and non-relapse mortality after HSCT. Methods have been developed for monitoring T cell alloimmune responses in vivo. Host reactive T cells are identified in short-term culture using CFSE staining and multi-parameter flow cytometry to monitor the frequencies of responding CD4 and CD8 cells. Effector cell function is further classified by intracellular staining for cytokines. The clonal variation of responding T cells is determined by spectratyping of T cell receptor (TCR) V-beta gene rearrangements. Donor T cell responses to host alloantigen is highly diverse. Many clones appear for a limited period of time suggesting that sequential activation and deletion may be occurring. Other clones persist for several months to years following transplantation. A high rate of spontaneous apoptosis is found among peripheral blood T cells during the first few months following transplantation primarily among T cells expressing activation markers. This activation induced cell death is not random but appears to be biased at the clonal level. Apoptosis occurs more frequently in HLA-mismatched transplants and patients with severe GVHD. Apoptosis decreases with administration of cortical steroids but increases again in patients who subsequently fail treatment or become steroid dependent. Studies are underway to further understand how T cell activation leads to apoptosis and whether or not this phenomenon simply represents clonal deletion. A better understanding of these in vivo reactions could lead to improved strategies for monitoring the immune suppression therapy facilitating tolerance induction and improving immune reconstitution.