To clarify the origin and function of the microglia residing in the central nervous system, we cloned brain cells from newborn and adult mice in soft agar containing the macrophage-specific growth factor, colony-stimulating factor-1 and expanded the cells from individual colonies in liquid culture medium. The results of molecular, immunophenotypic and functional analyses showed that the clones consisted of microglia derived from the macrophage family of cells. For instance, the microglia contain mRNA transcripts for the receptor for colony-stimulating factor-1 and truncated CD4 transcripts similar to those found in mouse macrophages but not T helper cells. About a third of the microglial progenitors gave rise to progeny that constitutively induced the selective proliferation of naive allogeneic CD8+ T cells in a CD4+ T cell-independent manner, a response that was inhibited by monoclonal antibodies to major histocompatibility complex (MHC) class I molecules on the microglia. Since all microglia expressed similar levels of MHC class I molecules, the basis for the alloantigen presentation likely resides in the ability of some clones of microglia to synthesize co-stimulator molecules that are required for CD8+ T cell proliferation. Thus, at least some microglia in mouse brain arise from endogenous progenitors and appear capable of specialized functions.

Natural suppressor (NS) cells, which nonspecifically suppress immune responses, are generally found at sites of hemopoietic generation or regeneration. Murine bone marrow NS cells were activated by recombinant interleukin 3 (rIL-3) or recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) and produced a soluble suppressor factor. In the present study, the soluble suppressor factor from bone marrow NS cells was found to be a potent inhibitor of myeloid colony formation at concentrations below those required for immunosuppression. NS cell supernatants inhibited the growth of granulocyte-macrophage colony-forming units (CFU-GM), granulocyte erythrocyte macrophage megakaryocyte colony-forming units (CFU-GEMM), and erythroid colony-forming units (CFU-E) to a similar extent. Neutralizing anti-transforming growth factor beta (TGF-beta) reversed the suppressive effects of the supernatants, suggesting that TGF-beta was involved in the suppression. The NS cell supernatants also inhibited the production of colony-stimulating activity by bone marrow stromal cells and the transcription of GM-CSF mRNA by activated T cells. These data suggest that NS cells are important regulators of hemopoiesis. NS cells, which are nonadherent, radioresistant non-T cells resident in the bone marrow, were shown to be sensitive to treatment with the lysosomotropic agent, L-leucine methyl ester, suggesting that the NS cells may be of large granular lymphocytic or monocytic lineage. Cytotoxicity studies revealed that cells in the NS population had natural cytotoxic (NC), but not natural killer (NK) activity.

We previously reported that murine bone marrow cells activated by interleukin-3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF) had potent nonspecific natural suppressor (NS) cell activity. In the present study, we demonstrated that these activated NS cells released a soluble factor (or factors) capable of nonspecifically inhibiting T cell mitogenic responses. Consistent with the properties of transforming growth factor-beta (TGF-beta), treatment of the NS supernates with heat failed to denature the factor, and in fact significantly increased its suppressive activity. The NS suppressor factor strongly inhibited proliferation of the TGF-beta-sensitive tumor cell line, A549. Cytokine activation of suppressive activity correlated with the production of a 10- to 13-kDa protein, consistent with the size of TGF-beta and rIL-3 induced a sevenfold increase in TGF-beta transcription. Finally, neutralizing anti-TGF-beta antibody inhibited the suppressive activity of the supernates, indicating that TGF-beta was responsible for most, if not all, of the suppression expressed by these bone marrow NS cells.

Mouse macrophages do not express CD4 on their surfaces. We used the polymerase chain reaction to investigate CD4 gene transcription in individual clones of primary mouse splenic macrophages and cell lines of spleen and bone marrow macrophages. The results show only the presence of CD4 mRNA transcripts that are truncated in the 3' coding sequence, thus explaining the lack of expression of a mature CD4 gene product by these cells.

Natural suppressor (NS) cells, which nonspecifically suppress immune responses, are present in the spleen following exposure to radiation, chronic graft-versus-host disease, or cancer and in normal bone marrow. A model system is described which allows the study of cytokines activating and inhibiting NS cells, cytokines mediating NS activity, and NS effects on cytokine synthesis. Recombinant interleukin-3 (rIL-3) and granulocyte-macrophage colony-stimulating factor (rGM-CSF) efficiently activated NS cells present in normal bone marrow and were effective at concentrations as low as 5 U/ml. At high concentrations, GM-CSF, but not IL-3, did not activate NS cells. Recombinant interferon-gamma (rIFN-gamma) blocked the activation of bone marrow NS cells by rIL-3, but did not down-regulate NS cells once activated. The NS cells secreted one or more soluble suppressor factors, which blocked IL-2 synthesis and also inhibited IL-2-dependent T cell proliferation in the presence of excess IL-2.

A panel of seven mouse splenic macrophage cell lines, derived from cloned progenitors, was compared for their ability to present antigen to Th1 or Th2 helper T cell lines and hybridomas, as well as to naive T cells, and to provide accessory cell function for the synthesis of antibody from primed B cells. One of the cell lines expressed MHC class II molecules and was the only line with constitutive antigen-presenting activity for Th1 cells. It may represent a subset of splenic macrophages responsible for the activation of naive Th1 helper cells in situ. The remaining six cell lines responded to INF-γ by up-regulating their class II expression and acquiring Th1 antigenpresenting activity. They may represent cells which, in situ, lack constitutive antigen-presenting activity but are promoted to presenting status by Th1-derived INF-γ. Five of the cell lines provided accessory cell function to Th2 cells, as indicated by antibody synthesis in suspensions of spleen cells from primed mice depleted of their antigen-presenting cells. One of the cell lines lacking accessory cell activity had constitutive antigen-presenting activity for Th1 cells. This reciprocal expression of antigen-presenting activity supports the idea that Th1 and Th2 helper cells are activated by different antigen-presenting cells. Finally, the cell lines differed in their ability to constitutively induce an allogeneic response; a response that was limited to CD8+ T cells occurred in a CD4+ helper cell-independent manner and was unaffected by the addition of INF-γ. The alloantigenpresenting macrophage cell lines also possessed the most efficient accessory cell activity for antibody synthesis. These cell lines, which represent a spectrum of antigen-presenting activities in the spleen afford models for defining the roles of macrophages in the induction of immune responses and for resolving issues concerning their development.