Within the hemopoietic system, LAR phosphatase is a T cell lineage-specific adhesion receptor-like protein whose phosphatase activity appears dispensable for T cell development, repertoire selection and function

Expression of the receptor-type tyrosine phosphatase LAR was studied in cel ls of the murine hemopoietic system. The gene is expressed in all cells of the Tcell lineage but not in cells of any other hemopoietic lineage and the level of expression in T cells is developmentally regulated. The CD4(-)8(- )44(+) early thymic immigrants and mature (CD4(+)8(-)/CD4(-)8(+)) thymocyte s and T cells express low levels, whereas immature (CD4(-)8(-)44(-) and CD4 (+)8(+)) thymocytes express high levels of LAR. Among bone marrow cells onl y uncommitted c-kit(+)B220(+)CD19(-) precursors, but not B cell lineage com mitted c-kit(+)B220(+)CD19(+) precursors, express low levels of LAR. In con trast to the c-kit(+)B220(+)CD19(+) pre-BI cells from normal mice, counterp arts of pre-BI cells from PAX-5-deficient mice express LAR, indicating that PAX-5-mediated commitment to the B cell lineage results in suppression of LAR. During differentiation of PAX-5-deficient pre-BI cell line into non-T cell lineages, expression of LAR is switched off, but it is up-regulated du ring differentiation into thymocytes. Thus, within the hemopoietic system, LAR appears to be a T cell lineage-specific receptor-type phosphatase. Howe ver, surprisingly, truncation of its phosphatase domains has no obvious eff ect on T cell development, repertoire selection or function.