SKEWING OF THE T CELL REPERTOIRE TOWARD A TARGET ANTIGEN DURING A PERIOD OF T CELL IMMUNE RECONSTITUTION. • 935

Vaccine based immunotherapy directed toward recently-defined tumor antigens represents a new approach for the treatment of neoplastic disease. Because minimal tumor burdens are most susceptible to such therapy, it is important to define how this approach could be used successfully in hosts immediately following debulking chemotherapy. Previously, we studied immune reconstitution in patients aged 1-24 years after intensive T cell depleting chemotherapy and found that thymic involution occurred in all patients studied immediately post-chemotherapy. For patients over 10 years of age, thymic inactivity persisted for at least 6 months after completion of chemotherapy. Hence, even relatively young patients appear dependent upon thymic-independent pathways of T cell regeneration for a prolonged period post-chemotherapy. Using thymectomized mice, we now have studied thymic-independent pathways of T cell regeneration after in vivo T cell depletion. We report that thymic-independent T cell regeneration primarily involves expansion of peripheral T cell populations and requires T cell-MHC interactions since CD8 expansion did not occur in β2m-/- mice (which lack MHC Class I) and CD4 expansion was diminished in mice treated with MHC Class II-blocking antibodies. When CD4 and CD8 T cells bearing transgenic T cell receptors (TCRs) were used as T cell inocula, expansion occurred only when antigen specific for the transgenic TCR was supplied. These results provide evidence that thymic-independent T cell regeneration occurs primarily via antigen-driven expansion of peripheral T cell populations. Such antigen-driven expansion of T cells bearing transgenic TCRs resulted in a marked skewing of the regenerated T cell repertoire toward the particular target antigens with approximately 40% of the regenerated CD4 or CD8 T cells expressing an antigen-specific TCR. Thus, when a T cell inoculum containing appropriate TCR specificities is supplied in combination with the target antigen during a period of T cell regeneration in thymic-deficient hosts, significant skewing of the T cell repertoire toward a particular target antigen can occur. We are using such an approach as the basis for a clinical trial of vaccine-driven expansion of antigen-specific T cells directed toward tumor-specific proteins expressed by Ewing's sarcoma and alveolar rhabdomyosarcoma which will follow a debulking phase of intensive cytotoxic chemotherapy.