Bacterial lipopolysaccharide (LPS) I has among its broad spectrum of immunologic activities the capacity to modulate the induction of a specific state of tolerance in mice to the thymus-dependent antigen human IgG (HGG), into a specific state of immunity to HGG (1). Mice treated with LPS shortly after the injection of a tolerogenic dose of deaggregated HGG (DHGG) not only fail to become tolerant to HGG (2), but demonstrate a delayed primary response to HGG, and also respond anamnestically to a subsequent immunogenic challenge of aggregated HGG (AHGG) (1). This phenomenon, which has been viewed as a very stringent test of an adjuvant effect (3), was originally described by Claman (4) with bovine gamma globulin tolerance in mice, and has also been seen more recently by Ornellas et al. (5) with sheep gamma globulin tolerance in rats.Recent studies have been directed toward precisely defining the cellular basis of this modulatory effect of LPS on HGG tolerance. It was found that the ' immune response to HGG which is seen as the result of dual treatment of mice with DHGG and LPS appears to occur in spite of the normal induction of tolerance in both HGG-specific thymocytes (1) and peripheral T cells (6). These observations suggest that LPS not only prevents tolerance induction in B cells, but also overcomes the normal requirement for HGG-specific T-helper cells. Furthermore, a positive correlation exists between the capacity of LPS to * This is Publication no.

There exists a unique strain of mouse, the C3H/HeJ, that is inherently refractive to many ofthe diverse biological effects ofbacterial lipopolysaccharide (LPS)l which are characteristically displayed by LPS in other strains of mice. For example, the C3H/HeJ mouse is refractory to the capacity of LPS to nonspecifically activate B lymphocytes (1, 2) as evidenced by the inability of spleen cells from this strain to support either LPS-induced mitogenic (3-6) or polyclonal antibody (6, 7) responses in vitro. This defect in the C3H/HeJ appears to be specific to LPS, since other B-cell mitogens, such as purified protein derivative of tuberculin (PPD), dextran sulfate, and poly I sponsor quantitatively normal mitogenic responses in this strain (3, 6, 7). The normally potent adjuvant effect of LPS on the immune response in vivo (8, 9) is also not seen in the C3H/HeJ mouse, as shown by the inability of LPS in this strain to enhance the antibody response to soluble bovine serum albumin (BSA) or to modulate the induction of a specific state of tolerance induced by deaggregated human IgG (HGG) into a specific state of immunity to HGG (5). The lack of these adjuvant effects in the C3H/HeJ mouse is not due to its inability to recognize the antigens, BSA and HGG, since the C3H/HeJ responds as well as other mouse strains when chal-* This is publication no . 1,017 from the

The C3a fragment of the third component of complement was found to have immunosuppressive properties. C3a is capable of suppressing both specific and polyclonal antibody responses. In contrast, C3a had no effect on antigen- or mitogen-induced B or T cell proliferative responses. The carboxy-terminal arginine is essential for C3a to exhibit its immunosuppressive properties. The serum carboxypeptidase inhibitor 2-mercaptomethyl-5-guanodinopentanoic acid, which prevents cleavage of the terminal arginine that would produce C3ades Arg-77, allowed us to assay the effects of C3a on in vitro immune response systems where serum is required. When the terminal arginine is removed from C3a, the resulting C3ades Arg-77 molecule is nonsuppressive. Helper T lymphocytes are the target of C3a-mediated suppression of the immune response. Substitution of T cells by soluble T cell factors was found to abrogate the C3a suppressive activity.

The lipopolysaccharide (LPS) 1 component of the cell wall of gram-negative bacteria possesses a broad range of biological activities and has been utilized to probe both subcellular and cellular phenomena (1-6). It is known to stimulate cell-specific activity in a diversity of cell types, including platelets (7), polymorphonuclear leukocytes (8), lymphocytes (9), fibroblasts (10), and macrophages (11) among others. Activation of lymphocytes has been studied in a number of systems: LPS has been shown to be a B-lymphocyte mitogen (12), a polyclonal B-lymphocyte activator (13), and an in vivo adjuvant (14) enhancing the antibody response to simultaneously administered antigens. Work by Claman (15) established that administration of LPS after tolerogen interfered with the in vivo induction of tolerance. Golub and Weigle (16) subsequently defined the temporal relationship between the injection of tolerogen and LPS. Moreover, Louis, et al. (17) showed that this regimen modulated the unresponsive state to one of immunity, and that this effect was limited to B cells, whose ensuing secondary response was T-independent in the face of continuing T-cell tolerance. Chiller and Weigle (18) demonstrated that LPS could be used to terminate tolerance to human gamma globulin (HGG) late in tolerance, at a time when T cells were still tolerant but B cells were responsive. Under these circumstances,

The ability of T lymphocytes to regulate the humoral immune response has been appreciated since Claman et al. (1) and Miller and Mitchell (2) described the absolute requirement for T helper cells in order for B cells to generate an antigen-specific response. Subsequently, modulation of the humoral response by T suppressor cells (3, 4) and T amplifier cells (5, 6) has been studied extensively. These regulatory cells have been characterized with respect to cell surface phenotype (7,8) and physical characteristics (9); soluble factors have been obtained from both helper and suppressor cells that replace the function of T cells in the regulation of antigen-specific humoral responses (10, 11).The regulatory effects of T lymphocytes on the nonspecific responses of B cells evoked by polyclonal B cell activators are much less thoroughly understood. Interest in these substances stems from the fact that the immune system of the body is frequently confronted with such activators in many infectious disease situations; moreover, polyclonal activation is thought to play a role in the pathogenesis of certain diseases of autoimmunity, such as systemic lupus erythymatosus. Although pokeweed mitogen is unable to evoke a polyclonal response from human B cells in the absence ofT cells (12,13), other mitogens such as protein A from Staphylococcus aureus stimulate polyclonal antibody production in the absence ofT cells (14). An analogous situation pertains to murine splenic B lymphocytes, which are able to respond to most polyclonat stimulants in the total absence of T cells (15). Because of the ubiquitous nature of such stimulants and the unusually large proportion of B lymphocytes that respond to them (16), it appeared probable that this arm of the humorai immune system is subject to some type of regulatory influence.Recent studies from this laboratory have demonstrated that addition of murine splenic T lymphocytes to B cell cultures results in marked enhancement of the polyclonal response to bacterial lipopolysaccharide (LPS) 1 (17, 18). The T cells *