This paper describes the further characterization of the T cell activity that is induced by allogeneic MHC gene products and provides help to mouse spleen B cells in a primary in vitro response to sheep erythrocytes.

We have previously shown that the helper activity induced by antigens controlled by the K or D regions was dependent on the presence of an Ly123 cell and that a single cell, present in high frequency, was required. In the present paper we show that activity induced by a difference at H-2K is unaffected by adult thymectomy but is diminished by prior injection of anti thymocyte serum. The spleen cell population positively selected for Ly2 (Ly2 and Ly123 cells) and hence depleted of Ly1 cells retained full helper activity. Ly1 cells were inactive. In contrast, helper activity induced by differences at the whole haplotype or at Mls were present in the Ly1 population and reduced or missing from the Ly2 and Ly123 populations. We concluded that the helper activity induced by allogeneic K or D is the function of a long lived recirculating Ly123 cell present in high frequency. We were unable to find any evidence that the Ly123 matured into an Ly1 cell before performing its effector function. The Ly phenotype of the effector cell is thus more closely linked to the MHC subregion that is recognized than the T cell function performed. Possible interpretations of the findings are discussed.

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This is publication No. 136 from the Department of Cellular and Developmental Immunology, and No. 1941 from the Research Institute of Scripps Clinic. This work was supported by United States Public Health Service Grant AI 08795 and American Cancer Society IM-1L.

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