CD45 regulates thymocyte survival during development in fetal thymic organ culture.

The CD45 protein tyrosine phosphatase is essential for T cell development. Its external domain undergoes changes in glycosylation and isoform usage during thymocyte development, the consequences of which remain unknown. The contribution of this complex external domain to T cell development is unknown so we sought to examine the impact of CD45 engagement on T cell development. Treatment of wildtype fetal thymic organ cultures (FTOC) with certain CD45-specific monoclonal antibodies resulted in decreased thymocyte numbers specifically at the double positive and CD4 single positive stages of development. The decrease in thymocyte number correlated with increased annexinV staining, an early indicator of apoptotic death. In contrast, CD45-/- FTOC exhibited decreased cellularity and increased annexinV staining at all stages of development. Thymocyte selection, as assessed by CD5 expression on DP thymocytes, was not affected by anti-CD45 treatment, whereas it was impaired in FTOC from CD45-deficient mice. The decrease in cellularity was not due to impaired proliferation as neither anti-CD45 treatment nor CD45 deficiency resulted in decreased proliferation in FTOC. Thus, antibodies specific for the external domain of CD45 results in the selective impairment of thymocyte survival at specific stages of development, whereas CD45 expression appears to be required for survival at all stages of development. Our studies show that CD45 expressed on thymocytes is an important regulator of survival during T cell development.