Immature hematopoietic cells display selective requirements for adhesion- and degranulation-promoting adaptor protein in development and homeostatsis

Adhesion- and degranulation-promoting adaptor protein (ADAP) modulates T cell development and function and promotes TCR signaling. Regulation of ADAP protein expression during thymopoiesis and in development of other hematopoietic lineages has not been explored. Using intracellular staining, we detected ADAP protein in bone marrow lymphocyte precursors. Like its binding partner SH2-containing leukocyte phosphoprotein of 76 kDa, ADAP is dynamically regulated during thymocyte positive selection. ADAP is also found in unconventional thymocytes, including NKT, CD8 alpha alpha, and TCR gamma delta T cells. In peripheral T cells, ADAP is up-regulated after TCR stimulation and with acquisition of memory status. Although absent in splenic B cells, ADAP is present in pro-B cells, as well as in BM erythrocyte and myeloid progenitors. Studies with radiation chimeras show that ADAP is dispensable for NKT, CD8 alpha alpha and TCR gamma delta T cell development, while confirming that ADAP is required for optimal development of conventional TCR alpha beta T cells in the thymus. Interestingly, ADAP is necessary for CD8 alpha alpha homeostasis in the small intestinal epithelium, yet is dispensable for optimal reconstitution of splenic B cell populations. Our observations highlight the dynamic regulation of ADAP during T cell maturation and document expression patterns that suggest a possible role for ADAP in development of non-T hematopoietic lineages.