A procedure is described for both depleting and obtaining in pure form Igbearing cells from mouse lymphoid populations. The Ig-bearing cells are identified by rosetting and the rosettes separated from the nonrosetting lymphocytes by centrifugation on Isopaque/Ficoll. The rosettes sink with the red cells and the nonrosetting lymphocytes float. The procedure is > 99.5 % efficient at depleting mouse lymphoid populations of Ig-rosetting cells and is also > 97 % efficient at removing the Ig-bearing cells detected by radioautography. Furthermore, after lysis of the red cells by hypotonic shock the Ig-bearing cells are recovered in a highly pure form. The total recovery of white cells and rosettes applied is > 85 %.This procedure was shown t o produce a functional separation of T and B lymphocytes. The cell population depleted of Ig-rosettes behaved as a pure T cell preparation. It lacked precursors of antibody-forming cells, but contained virtually all of the @positive lymphocytes, the bulk of the helper cells detected in two in vitro hapten carrier antibody responses and all the cells which responded and produced cytotoxic cells in MLC. In contrast, the preparation of Ig rosettes expressed B cell properties. This population contained all of the antibody forming cell precursors, few helper cells and @positive lymphocytes and n o MLC-responding cells. However, there was some evidence that a small subpopulation of T cells exists which possesses surface Ig.The separation system was used t o formally demonstrate that carrier primed T cells collaborate with hapten primed B cells t o generate an anti-hapten antibody response t o a hapten-carrier conjugate. It was also established that in MLC responder B cells in no way collaborate with responder T cells t o generate cytotoxic cells.

The role of the H-2 gene complex in expression of cytotoxicity exerted by specific ectromelia-immune thymus-derived (T) cells against ectromelia-infected target cells was examined. A repertoire of inbred mouse strains (some congenic) including the H-2 haplotypes k, d, b, s, q, the recombinant H-2a(k/d) and F1 hybrids (k/b and d/b) were immunized with virus and their spleen cells tested 6 days later, at the peak of the primary response, against H-2k,H-2d and H-2b target cells. Significant specific cytotoxicity occurred only when the immune cell donors and the target cells shared all or part of the same H-2 gene complex. For example, H-2a (k/d) immune cells killed both H-2k and H-2d target cells. There was no detectable effect of the non-H-2 genetic background, H-2 public specificities, or the M-locus. Target cells infected with ectromelia virus exhibited quantitative or qualitative changes (or both) in expression of normal H-2 antigens as indicated by reduced susceptibility to killing by T cells activated against H-2 antigens in mixed lymphocyte culture. These data are consistent with the hypothesis that T cells in this system are responding to virus-induced, specific changes in antigens on infected cells which are controlled by genes in the H-2 complex; these genes seem likely to be those coding for H-2 private specificities, or genes closely linked to them.

Spleen cells from mice immunized intravenously with attenuated ectromelia virus were cytotoxic for target cells infected with virulent ectromelia virus in the absence of exogenous complement; cytotoxicity was measured by 51Cr release from target cells. L929 cells were the most sensitive target cells, but the effect could also be demonstrated with P‐815 mastocytoma cells and chick embryo cells; mouse embryo cells were unsatisfactory. Hyperimmune anti‐ectromelia serum and complement was not significantly cytotoxic although fluorescein‐conjugated antiserum stained virus‐infected L929 cells. Release of 51Cr label from ectromelia‐immune spleen cells (at a spleen cell: target cell ratio of 100: 1) increased in a linear manner with time, until a plateau was reached at 20–24 hours. Cytotoxicity appeared to be specific in that ectromelia‐immune spleen cells killed ectromelia‐infected L929 cells, but not uninfected L929 cells, whereas spleen cells from mice immunized with Listeria monocytogenes (a potent stimulus for cell‐mediated immunity) or from normal mice, did not kill ectromelia‐infected L929 cells. Spleen cells from mice immunized with vaccinia virus (a poxvirus closely related to ectromelia) were cytotoxic for ectromelia‐infected L929 cells. Spleen cells from mice immunized with ectromelia virus had acquired cytotoxic activity by 2 days after immunization. Cytotoxic potency reached a peak at 6 days and had declined to a low level by day 10. The potential significance of the phenomenon in relation to control of viral infection is discussed.

Lewis rats that are primed with guinea pig spinal cord homogenized in complete Freund's adjuvant (GPSCH-CFA) develop overt symptoms of experimental allergic encephalomyelitis (EAE). Treatment with the iron-chelating agent, desferrioxamine B mesylate (DFOM), at various times before the onset of EAE, dramatically suppressed both the severity and duration of disease. When DFOM was administered to rats soon after the development of neurological signs, a rapid recovery occurred, though mild, transient symptoms could be seen approximately 1 wk after withdrawal of the drug. Treatment with DFOM was always accompanied by a diminution of T cell responsiveness on the part of the delayed-type hypersensitivity/helper subset and, on histological examination, an absence of inflammatory cells from lesions. Iron is believed to influence both the migration and function of immune effector cells. It can also act as a catalyst in the formation of free radicals, which are highly toxic agents causing tissue damage in sites of inflammation. The mechanisms underlying the effect of DFOM on the severity of EAE, and the possible implications for treatment of multiple sclerosis are discussed.

Cell‐mediated cytotoxicity of ectromelia‐immune spleen cells against ectromelia‐infected L929 target cells was abrogated by treatment of the spleen cells with anti‐Θ antibody and complement. Treatment of spleen cells with anti‐immunoglobulin serum and complement, or removal of macrophages had little or no effect on cytotoxicity. Specific or nonspecific soluble cytotoxic factors were not detected, and exogenous interferon did not enhance cytotoxicity. No blocking of cytotoxicity was detected with either hyperimmune or 9‐day immune anti‐ectromelia serum. By varying the spleen cell‐target cell ratio from 3: 1 to 400: 1, it was shown that the efficiency of cytotoxicity was inversely related to spleen cell density. These results were interpreted to mean that thymus‐derived (T) cells, probably acting alone, were responsible for cytotoxicity and that the mechanism involved contact between T cell and target cell.