Antibody mediated regulation of immune responses. I. Enhancement of specific antibody responses through IgM antibodies

Specific IgM and Regulation of Antibody Responses

Specific IgM, administered together with the antigen it recognizes, enhances primary antibody responses, formation of germinal centers, and priming for secondary antibody responses. The response to all epitopes on the antigen to which IgM binds is usually enhanced. IgM preferentially enhances responses to large antigens such as erythrocytes, malaria parasites, and keyhole limpet hemocyanine. In order for an effect to be seen, antigens must be administered in suboptimal concentrations and in close temporal relationship to the IgM. Enhancement is dependent on the ability of IgM to activate complement, but the lytic pathway is not required. Enhancement does not take place in mice lacking complement receptors 1 and 2 (CR1/2) suggesting that the role of IgM is to generate C3 split products, i.e., the ligands for CR1/2. In mice, these receptors are expressed on follicular dendritic cells (FDCs) and B cells. Optimal IgM-mediated enhancement requires that both cell types express CR1/2, but intermediate enhancement is seen when only FDCs express the receptors and low enhancement when only B cells express them. These observations imply that IgM-mediated enhancement works through several, non-mutually exclusive, pathways. Marginal zone B cells can transport IgM-antigen-complement complexes, bound to CR1/2, from the marginal zone and deposit them onto FDCs. In addition, co-crosslinking of the BCR and the CR2/CD19/CD81 co-receptor complex may enhance signaling to specific B cells, a mechanism likely to be involved in induction of early extrafollicular antibody responses.

Selective IgM Deficiency: Clinical and Laboratory Features of 17 Patients and a Review of the Literature

PURPOSE

Primary selective IgM deficiency (sIgMD) is a primary immunodeficiency with unclear pathogenesis and a low number of published cases.

METHODS

We reviewed clinical and laboratory manifestations of 17 sIgMD patients. Serum IgM, IgG, and its subclasses, IgA, IgE, antibodies against tetanus toxoid, pneumococcal polysaccharides and Haemophilus influenzae type b, isohemagglutinins, and T and B lymphocyte subsets, expressions of IgM on B cells and B lymphocyte production of IgM were compared with previously reported case reports and a small series of patients, which included 81 subjects in total.

RESULTS

We found that some patients in our cohort (OC) and published cases (PC) had increased IgE levels (OC 7/15; PC 21/37), decreased IgG4 levels (OC 5/14), very low titers of isohemagglutinins (OC 8/8; PC 18/21), increased transitional B cell counts (OC 8/9), decreased marginal zone B cell counts (OC 8/9), and increased 21^low B cell counts (OC 7/9). Compared with the PC (20/20), only two of five OC patients showed very low or undetectable production of IgM after stimulation. A majority of the patients had normal antibody production to protein and polysaccharide antigens, basic lymphocyte subset counts, and expression of surface IgM molecules on B cells.

CONCLUSIONS

Low IgM levels are associated with various immunopathological disorders; however, pathogenic mechanisms leading to decreased IgM serum level in selective IgM deficiency remain unclear. Moreover, it is difficult to elucidate how strong these associations are and if these immunopathological conditions are primary or secondary.

Intraclonal competition inhibits the formation of high-affinity antibody-secreting cells

Protective immunity requires a diverse, polyclonal B cell repertoire. We demonstrate that affinity maturation of the humoral response to a hapten is impaired when preexisting clonally restricted cells recognizing the hapten are dominant in the B cell repertoire. B1-8i(+/-) mice, which feature a high frequency of B cells with nitrophenyl (NP)-binding specificity, respond to NP-haptenated proteins with the production of NP-specific Abs, but affinity maturation is impaired due to insufficient generation of high-affinity Ab-producing cells. We manipulated the frequency of NP-specific B cells by adoptive transfer of B1-8 B cells into naive, wild-type recipients. Remarkably, when 10(4) B1-8 B cells were transferred, these cells supported efficient affinity maturation and plasma cell differentiation. In contrast, when 10(6) B1-8 cells were transferred, affinity maturation did not occur. These data indicate that restricting the frequency of clonally related B cells is required to support affinity maturation.

Regulation of antibody responses via antibodies, complement, and Fc receptors

Antibodies can completely suppress or enhance the antibody response to their specific antigen by several hundredfold. Immunoglobulin M (IgM) enhances antibody responses via the complement system, and complement activation by IgM probably starts the chain of events leading to antibody responses to suboptimal antigen doses. IgG can enhance primary antibody responses in the absence of the complement system and seems to be dependent on Fc receptors for IgG (FcgammaRs). IgE enhances antibody responses via the low-affinity receptor for IgE (FcepsilonRII/CD23). The precise effector mechanisms that cause enhancement are not known, but direct B-cell signaling, antigen presentation, and increased follicular localization are all possibilities. IgG, IgE, and IgM may also suppress antibody responses when used in certain immunization regimes, and it seems reasonable that an important mechanism behind suppression is the masking of antigenic epitopes by antibodies. In addition, FcgammaRIIB, which contains a cytoplasmic inhibitory motif, acts as a negative regulator of antibody responses. This receptor, however, may prevent the antibody responses from exceeding a certain level rather than causing complete suppression.

Antibody-mediated modulation of the immune response

We have studied the ability of monoclonal IgM and IgG antibodies to enhance or suppress immune responses and attempted to dissect the underlying mechanisms. Both IgM and IgG1 antibodies increased the rate of clearance of antigen from the circulation. Monoclonal IgM antibody to SRBC was found to specifically increase antibody responses, enhancement being insensitive to low doses of irradiation (150 R). IgM antibody specifically depressed the delayed hypersensitivity response to SRBC in vivo. Following administration of IgM in vivo, in vitro responses to SRBC were also enhanced. This in vitro enhancement appeared to depend on both T cells and B cells. In contrast, monoclonal IgG1 antibody to SRBC specifically depressed antibody responses in vivo. Such depressed antibody responses were also seen in vitro following IgG1 in vivo and did not appear to be due to the induction of suppressor T cells.

Detection in vivo 6 hours after immunization of a mediator with possible antisuppressor activity

Immunization of mice with foreign proteins or particulate antigen induces the formation of immunoglobulin (Ig)-antigen complexes which are strongly cytophilic for T cells and have been shown recently to markedly enhance the 7 S antibody response. In this report we demonstrate that pretreatment of the animals with cyclophosphamide or anti-I-J antiserum eliminates the difference in the antibody response between the mice injected with the complexes and the controls, in other words, the enhancement. Six hours after allogeneic stimulation the serum of mice contains also a cytophilic (for T cells) Ig which most likely represents, as in the case of the foreign antigens, complexes of Ig with alloantigens. The allogeneically induced 6-h serum (6HS) contains a factor which enhances the cytotoxic T lymphocyte(s) (CTL) response in vivo. As with the 7S antibody response, pretreatment of mice with cyclophosphamide, in doses known to eliminate suppressor cell expression, "masks" the CTL enhancement of the allogeneically induced 6HS. The same result was also observed with the anti-I-J antiserum. In conclusion, the 6-h complexes in cyclophosphamide- and anti-I-J-treated mice do not produce a 7 S antibody or CTL response above that produced by the control group (no complexes). Using heat-treated allogeneic tumor cells, known to induce suppressor cells in vivo, we have shown that for a low dose of tumor cells, the allogeneically induced 6HS did not block the induction of suppressor cells, but completely blocked their function. These results suggest that the enhancement of both humoral and cell-mediated immunity by the 6HS (complexes) is likely to be due to interference with normal suppressor cell function, that is, an antisuppressor mechanism.

Immunoregulation of the anti-bovine serum albumin response by polyclonal and monoclonal antibodies

Monoclonal or polyclonal antibodies directed toward determinants on limited structures of bovine serum albumin (BSA) (P505-582) were shown to regulate the entire anti-bovine serum albumin (BSA) immune response when passively administered to mice 24 hr prior to immunization. Regulation was observed as suppression of the humoral IgG immune response toward all BSA determinants except those on fragment P505-582. By Day 21 suppression of humoral response was most pronounced toward determinants present on the carboxy terminal end of the molecule (N 307-582). These observations demonstrate that monoclonal antibodies directed against a single determinant on a protein molecule have the capacity to regulate the immune response to a multiplicity of determinants present on the same protein. The data lend support to concepts of antibody-induced regulation by induction of suppressor cells or idiotype recognition.

Mechanisms of autoimmunity: a role for cross-reactive idiotypes

Ehrlich was rarely given to trivial pronouncements and his recognition of the central importance of the distinction between self and non-self by the immune system, embodied in his concept of 'horror autotoxicus'(1), is no exception. This is despite the apparent paradox of the idiotype network in which antibodies recognize self-epitopes on other antibody molecules or antigen receptors as part of the normal process of immune regulation. In this review Anne Cooke and her colleagues examine the possible factors which may contribute to the breakdown of self-tolerance and the establishment of autoimmune states.

Specific monoclonal IgM is a potent adjuvant in murine malaria vaccination

Recent experiments in the murine system have indicated that the passive acquisition by offspring of maternal anti-malarial IgG antibodies while conferring some degree of immunity against a primary infection, paradoxically prevents the generation of acquired immunity through vaccination. Therefore, in view of earlier findings concerning the competitive effects of specific IgM and IgG antibodies, we investigated whether specific monoclonal IgM antibodies could be used to potentiate the response to a blood-stage murine malaria vaccine. We now report that small amounts of purified monoclonal anti-parasite IgM can specifically potentiate both priming and memory cell generation in response to vaccination as evidenced by survival after infection, and that the magnitude of this effect is greater than that found with a more conventional nonspecific adjuvant (Bordetella pertussis). Additionally, in offspring of immune mothers, where vaccination is ineffective for up to 8 weeks due to the presence of maternal IgG, we have found that IgM when administered with the vaccine can completely overcome this inhibition by its adjuvant effect.