Spleen cells from C57BL/6J mice bearing Ehrlich carcinoma growing as a solid tumor show progressive unresponsiveness to concanavalin A (Con A) and lipopolysaccharide (LPS) mitogens. This is accompanied by striking spleen enlargement with marked hematopoietic activity. Lymphoproliferative assays of normal spleen cells in co-culture with tumor-bearing spleen cells (TBSC) show that: (a) TBSC contain non-specific suppressor cells able to abrogate both Con A and LPS responses, or mixed lymphocyte reaction, of normal spleen cells and (b) suppression by TBSC is MHC-unrestricted, non-prostaglandin-mediated and greatly enhanced by Con A supernatants. Suppressor cells associated with TBSC are large, low-density cells without markers of mature B or T lymphocytes or of the mononuclear phagocyte system. Most appear to be asialo-GM1-negative, as suppression was only partially inhibited by treatment with anti-asialo-GM1 and complement. Since NK activity is lacking in TBSC, our data strongly suggest that these "null" suppressor cells are related to the natural suppressor (NS) cells found described in normal bone-marrow and neonatal spleens, or induced in adult spleens by total lymphoid irradiation, graft-vs.-host disease, or cyclophosphamide treatment.

Although reagents are available to block mouse complement receptor type 2 and/or type1 (CR2/CR1, CD21/CD35) function in acute or short term models of human disease, a mouse anti-rat antibody response limits their use in chronic models. We have addressed this problem by generating in Cr2−/− mice a mouse monoclonal antibody (mAb 4B2) to mouse CR2/CR1. The binding of murine mAb 4B2 to CR2/CR1 directly blocked C3dg (C3d) ligand binding. In vivo injection of mAb 4B2 induced substantial down regulation of CR2 and CR1 from the B cell surface, an effect that lasted six weeks after a single injection of 2 mg of mAb. The 4B2 mAb was studied in vivo for the capability to affect immunological responses to model antigens. Pre-injection of mAb 4B2 before immunization of C57BL/6 mice reduced the IgG1 antibody response to the T-dependent antigen sheep red blood cells (SRBC) to a level comparable to that found in Cr2−/− mice. We also used the collagen-induced arthritis (CIA) model, a CR2/CR1-dependent autoimmune disease model, and found that mice pre-injected with mAb 4B2 demonstrated substantially reduced levels of pathogenic IgG2a antibodies to both the bovine type II collagen (CII) used to induce arthritis and to endogenous mouse CII. Consistent with this result, mice pre-injected with mAb 4B2 demonstrated only very mild arthritis. This reduction in disease, together with published data in CII-immunized Cr2−/− mice, confirm both that the arthritis development depends on CR2/CR1 receptors and that mAb 4B2 can be used to induce biologically relevant receptor blockade. Thus mAb 4B2 is an excellent candidate for use in chronic murine models to determine how receptor blockage at different points modifies disease activity and autoantibody responses.

C57BL/6J mice immunized with devitalized Ehrlich tumor (ET) cells produce high serum levels of IgM antibodies to ET cell-surface carbohydrates that are critical in the observed resistance against this tumor. However, this response is not found in ET-bearing mice at any stage of tumor development. Since previous studies had shown splenic natural suppressor (NS) cells in ET-bearers, their role in such IgM impairment was assessed. Here we show that tumor-bearers' spleen cells (TBSC) are unable to produce IgM in vitro in response to LPS, due to the presence of NS cells. Nevertheless, TBSC do produce IgM antibodies to ET cell-surface carbohydrates in increasing amounts as the tumor progresses. Yet these antibodies are not detected in sera of ET-bearers and are greatly decreased in immunized mice with a growing tumor. Moreover, increasing amounts of circulating carbohydrates, able to absorb most specific IgM, are found in ET-bearing sera associated with a large molecular size structure(s). These carbohydrates are also found in ET cell-culture supernatants and cell-free ascites fluid derived from this tumor, indicating their tumor origin. Taken together, our results indicate that lack of specific IgM antibodies in ET-bearing mice is not due to faulty production, but to in vivo absorption by carbohydrates shed from ET cells in increasing amounts as the tumor progresses. Thus, NS cells are unable to suppress this IgM production in vivo, despite the strong suppressor activity they show for many responses in vitro.

Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional cytokine that promotes IgA/IgG2b switching and secretion. Here, we show a differential effect of TGF-beta 1 on Ig production by lipopolysaccharide-stimulated spleen and lymph node (LN) B cells. Exogenous TGF-beta 1 increased IgA production in B cell cultures and IgG2b production by spleen B cells. In contrast, IgG2b was suppressed by TGF-beta 1 in cultures of LN B cells, although endogenous TFG-beta was required for IgG2b production in LN B cell cultures. The suppressor properties of exogenous TGF-beta 1 (0.5 ng/ml) on IgG2b production by LN B cells were also seen when testing IgG1 or IgG2a induced by interleukin-4 or interferon-gama, respectively. These differences between B cells from each lymphoid tissue appeared to be related to a different TGF-beta antiproliferative effect, since proliferation of LN B cells was extremely sensitive to TFG-beta 1 and IgG2b production was more sensitive than IgA to the TFG-beta-mediated suppression. However, by counteracting the antiproliferative effect of TGF-beta 1 with a CD40 agonistic mAb (IC10), the IgG2b response by LN B cells was still lacking. IC10 was nevertheless inhibitory for IgG2b production in most cases, while increasing secretion of IgA in the very same cultures. Taken together, the results suggest that functional differences between spleen and LN B cells do exit, at least with regard to the immunomodulating properties of TGF-beta on both proliferation and Ig production. Moreover, functional differences exist between cells committed for IgA and IgG2b regarding their sensitivity to the antiproliferative activity of TGF-beta 1 and the effect of CD40-derived signals on Ig secretion.

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