thymocytes (C57BL) restored the immune response of T celldeprived (C3H x C57BL)F, cultures t o control levels. The same principle held true in all combinations tested. Allogeneic cells other than thymocytes (brain, kidney) carrying the same major surface antigens did not stimulate t h e immune response t o SRBC in anti@ treated cultures. Likewise, allogeneic t hymocytes lysed by freezing and thawing, were quite ineffective.We thank Dr. George Klein, Stockholm, for kindly having supplied us with some of the heterozygous and homozygous mice, Dr. K.
Rajewsky, Cologne, for reference samples of anti-@ serum, Dr. V. Hammerling, Giessen, for CS7BL mice and Dr. F. Bach, Madison, for his valuable help in preparing the manuscript. We gratefully acknowledge the expert technical assistance of Miss L. Hink and Mrs. I. Wolj: This work was partially supported by the Deutsche Forschungsgemeinschaft and the Stiftung Volkswagenwerk.Received May 26, 1971.
Discussion
ReferencesSeveral ways exist t o explain reconstitution b y allogeneic cells. We would like t o cite three, not necessarily mutually exclusive, possibilities. First, in the presence of allogeneic B cells the immature cells may be eliminated in v i h o , as they are by hydrocortisone in vivo. Alternatively, the possible attraction of the allogeneic spleen cells towards t h e added thymocytes may overcome the crowding and interference by the immature cells. Second, t h e immature thymocytes may be stimulated by t h e antigens on the surface of the allogeneic B spleen cells. This may lead t o the maturation of some immature thymocytes and thus increase the number of mature T cells which can cooperate effectively. Third, it seems possible that with the allogeneic cells interacting, a potentiating factor is produced [8] which amplifies the collaboration. Mitomycin C treated cells can still elaborate such a response [9]; our irradiation results would thus b e consistent with such a possibility. The latter two explanations could also be more easily reconciled with the increased response, beyond control values, seen in many experiments.Our experiments show that, under appropriate conditions, normal thymocytes are able t o collaborate in yitro with B cells in the response t o SRBC.
