Previous studies have demonstrated that I-J-subregion-controlled Ia antigens are only expressed on a small subpopulation of peripheral T lymphocytes which includes the suppressor T cells of antibody responses (6). This subpopulation of T cells cannot be detected by conventional dye-exclusion cytotoxicity tests. A sensitive rosetting procedure therefore was developed for detecting the binding of anti-Ia antibodies to T lymphocytes. This assay system, unlike the complement lysis technique, has a low background and since it represents a direct binding assay could detect noncomplement-fixing antibodies in the antisera.

Anti-Ia sera were absorbed with B cells and using the rosetting procedure in genetic mapping studies the remaining antibodies were found to be directed against I-J-subregion-controlled determinants. These determinants were shown to be highly haplotype specific for H-2(k) and H-2(s) and appeared to be exclusively expressed on Ly-l.l(-), Ly2.1(+), T lymphocytes, at least some of which were suppressor T cells. Lymphoid organs differed in their content of anti-I-J-reactive cells, the hierarchy being spleen, lymph node more than thymus, bone marrow. In contrast, on a T-cell basis, a high proportion (35 percent) of the T cells in bone marrow reacted with anti-I-J antibodies, a substantial proportion (13 percent) of T cells from spleen were reactive, whereas the lymph node and thymus T-cell populations contained only a small proportion of positive cells (1-4 percent).

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