Mice injected with the bone-seeking isotope, 89Sr, were depleted of marrow erythropoietic progenitor cells but maintained normal or near-normal numbers of splenic erythropoietic progenitor cells, lymphoid tissue alloantigen-sensitive units responsible for graft-vs-host reactions, and splenic antigen-sensitive units responsive to sheep erythrocytes which give rise to antibody-producing cells. The ability of such mice to reject marrow allografts was suppressed. It is concluded that a separate class of marrow-dependent immunocompetent cells exist, now called “M cells,” which are responsible for rejection of allogeneic dispersed hemopoietic cells.

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Supported in part by Contract NIH 70-2035 with Chemotherapy, National Cancer Institute, Grant IN-54 from the American Cancer Society and Grant GB35852 from the National Science Foundation.

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