Requirement for ROR gamma in thymocyte survival and lymphoid organ development

Most developing thymocytes undergo apoptosis because they cannot interact p roductively with molecules encoded by the major histocompatibility complex. Here, we show that mice lacking the orphan nuclear hormone receptor ROR ga mma lose thymic expression of the anti-apoptotic factor Bcl-xL. ROR gamma t hus regulates the survival of CD4(+)8(+) thymocytes and may control the tem poral window during which thymocytes can undergo positive selection. ROR ga mma was also required for development of lymph nodes and Peyer's patches, b ut not splenic follicles, In its absence, there was loss of a population of CD3(-)CD4(+)CD45(+) cells that normally express ROR gamma and that are Lik ely early progenitors of lymphoid organs. Hence, ROR gamma has critical fun ctions in T cell repertoire selection and Lymphoid organogenesis.