Recent findings on the structure and function of teleost IgT

As key effector molecules of jawed vertebrate’s adaptive immune system, immunoglobulins are produced by B lymphocytes, either as a secretory form (antibody) or as a membrane form (B cell receptor). Until recently, teleost fish B cells were thought to express only two classes of immunoglobulins, IgM and IgD. In addition, IgM in these species was thought to be the only immunoglobulin isotype responding to pathogens both in systemic or mucosal compartments. However, the unexpected discovery of IgT, a new teleost immunoglobulin unearthed in 2005, has provided for new opportunities to analyze further roles of teleost immunoglobulins in these two physiologically distinct compartments. The smoke about the potential function of IgT has cleared recently with the finding that this immunoglobulin appears to be specialized in gut mucosal immunity. Significantly, the new capability of measuring not only IgM but also IgT responses will greatly facilitate the evaluation and understanding of fish immune responses as well as the protective effects of fish vaccines. The purpose of this review is to summarize the molecular characterization of new IgT orthologs and subtypes in teleosts, as well as to describe the new findings concerning the protein structure of IgT, the B cells producing it, and its role in mucosal immunity.     

Differential immunosuppressive effects of ascites fluid from mastocytoma-bearing mice on primary vs secondary immunocyte responses.

The effects of immunosuppressive ascites fluids from mastocytoma-bearing mice on the primary vs secondary immune response to sheep red blood cells (SRBC) was examined. Injection of mice with ascites fluid from tumor-bearing mice markedly depressed the primary immune response of normal syngeneic mice challenged with SRBC. However, there was a preferential depression of the 19S IgM antibody response as compared with the 7S IgG response. Injection of ascites fluid shortly before secondary immunization of mice with SRBC also resulted in depressed IgM PFC responses but only a slight to moderate depression of IgG PFC. Treatment of mice with the ascites fluid before primary immunization had little if any effect on the secondary IgG PFC response, although the IgM response was moderately depressed. These results indicate that the immunosuppressive factor(s) present in the ascites fluid of mastocytoma-bearing mice has a differential effect on distinct classes of immunocytes. Those immunocytes or their precursors involved in formation of low efficiency 7S IgG antibody are more resistant to immunodepression. Such differences appear due to different sensitivities of cells involved in the immune response system.     

Fish immunoglobulins and the genes that encode them

The nature of fish antibodies (concentrating primarily on the most studied species of bony and cartilaginous fishes) is discussed in terms of their immunoglobulin biochemistry and immunobiology. The major serum immunoglobulin (IgM) is described in detail, and structural variants of IgM are discussed in terms of their distribution in different fish species, and different anatomical sites within a fish (e.g. blood, mucus, bile). Structural variation in IgM includes the size of the constituent heavy and light polypeptide chains, and the extent to which they are covalently associated with one another. The intramolecular heterogeneity of binding sites for antigen on IgM is reviewed and possible mechanisms for the phenomenon are presented. The evidence suggests that some, but not all, species of fish possess a detectable J chain in their IgM. The general nature of the fish immune response is that IgM antibody of moderate affinity is produced and prolonged or repeated immunization: (a) fails to produce a switch to production of a non-IgM class of antibody, and (b) fails to induce substantial increases in the affinity of the specific antibodies. Evidence supports a conclusion that fish lack the typical secondary antibody response seen in mammals, and possess antibodies of limited heterogeneity. Our current understanding of the genetic basis for fish antibodies is presented. Fish appear to utilize the same basic genetic elements as mammals to encode and regulate the expression of their immunoglobulins. The teleost heavy chain (IgH) locus resembles that of mammals and amphibia in its organization. The IgH locus of elasmobranchs is arranged in a unique multicluster organization. The light chain loci of elasmobranchs are organized analogously to the heavy chain locus (in multiclusters). The structure of the light chain locus of teleosts is presently unknown. Teleost fish utilize a unique pattern of RNA processing to generate the secreted and membrane receptor forms of the IgM heavy chain. The genes encoding the unique low molecular weight Ig heavy chain found in skates and rays have been cloned and sequenced, and also display the multicluster pattern of organization. Teleost fish appear to have normal numbers of variable regions: it is hypothesized (but as yet unproven) that the failure of their IgM to increase in affinity is due to a deficiency of somatic hypermutational mechanisms in their Ig gene variable regions during B lymphocyte differentiation.     

Antigen processing and presentation in teleost immune responses

Recent in vitro studies clearly demonstrate the importance of antigen processing and presentation in the generation of immune responses to T-dependent antigens (i.e. proteins and hapten-carrier conjugates) in phylogenetically lower vertebrates such as teleosts. Similar to the situation in mammals, antigens are processed and presented by accessory or antigen-presenting cells (APC), such as monocytes or macrophages, to specific lymphocytes in a seemingly alloantigen- (presumably major histocompatibility complex [MHC] or MHC-like) restricted fashion. Results show that processing involves proteolysis, which presumably occurs within acidic subcellular compartments. The requirement for processing can be circumvented by the presentation of peptide fragments of the native antigen on paraformaldehyde-fixed APC. Moreover, usage of structurally defined proteins, such as cytochrome C, as model antigens reveals that their species variants are cross-stimulatory to immune fish lymphocytes. Molecular analyses of such antigens reveal the existence of overlapping epitopes that seem to define the specificity of the immune response to such “families” of antigens but not to other unrelated (yet structurally defined) antigens. Consequently, these studies corroborate the hypothesis that immune functions in the divergent classes of vertebrates are highly conserved. Further, results from such studies also show that these immunologic processes appear to occur under low temperature regimes previously reported to suppress primary immune responses. Hence, these studies provide direct evidence that low temperature-induced immunosuppression in fish does not involve impaired APC functions. In light of the above observations indicating similarities between fish and mammalian systems, implications for fish vaccine design are also discussed.     

Temperature influence on Arctic charr (Salvelinus alpinus L.) antibody response to a cellular antigen

In order to elucidate the immune responses of Arctic charr in relation to temperature, groups were acclimated to a moderate (9°C) and a cold temperature regime (4°C), as well as subjected to a temperature decrease (from 9 to 4°C) immediately prior to an immunization with sheep red blood cells. The charr kept at 9°C responded with increased primary and secondary antibody titres, as seen by direct haemagglutination, while fish at 4°C, as well as the fish subjected to a temperature reduction, displayed lower and lowest antibody titres, respectively, and only after a second immunization. It is concluded that Arctic charr can respond to a cellular antigen with a humoral immune response typical for other teleosts, but that the immune response is delayed and diminished at low temperatures. This temperature-induced immune suppression is intensified if the fish have not been acclimated to cold water prior to immunization. Accepted: 10 October 1999     

The effects of carrier-specific helper T-cell tolerance on antibody avidity in the anti-hapten response.

Rats rendered tolerant to ultracentrifuged sheep γ-globulin (SGG) have been shown to make a poor anti-trinitrophenyl (TNP)-specific antibody response upon challenge with TNP-SGG in complete Freund's adjuvant (CFA). We have been able to use this carrier-tolerance system in studying specific helper T-cell unresponsiveness to IgG and IgM antibody responses. By using the plaque inhibition technique to measure antibody avidity, we found that there appears to be no difference in the avidity of antibody responses to TNP between the SGG-tolerant and control groups when both are challenged with TNP-SGG in CFA. This was found to be true in both the 19 and 7S antibody responses in vivo as well as in an adoptive transfer model. In addition, studies on the maturation of 19 and 7S antibody responses showed no differences in antibody avidity between carrier-tolerant and control groups. These findings imply that carrier-specific helper T cells do not play a controlling role in determining whether high- or low-avidity hapten-specific B-cell precursors will proliferate in response to challenge with a hapten-carrier conjugate.     

Initial B-cell responses to transmitted human immunodeficiency virus type 1: virion-binding immunoglobulin M (IgM) and IgG antibodies followed by plasma anti-gp41 antibodies with ineffective control of initial viremia

A window of opportunity for immune responses to extinguish human immunodeficiency virus type 1 (HIV-1) exists from the moment of transmission through establishment of the latent pool of HIV-1-infected cells. A critical time to study the initial immune responses to the transmitted/founder virus is the eclipse phase of HIV-1 infection (time from transmission to the first appearance of plasma virus), but, to date, this period has been logistically difficult to analyze. To probe B-cell responses immediately following HIV-1 transmission, we have determined envelope-specific antibody responses to autologous and consensus Envs in plasma donors from the United States for whom frequent plasma samples were available at time points immediately before, during, and after HIV-1 plasma viral load (VL) ramp-up in acute infection, and we have modeled the antibody effect on the kinetics of plasma viremia. The first detectable B-cell response was in the form of immune complexes 8 days after plasma virus detection, whereas the first free plasma anti-HIV-1 antibody was to gp41 and appeared 13 days after the appearance of plasma virus. In contrast, envelope gp120-specific antibodies were delayed an additional 14 days. Mathematical modeling of the earliest viral dynamics was performed to determine the impact of antibody on HIV replication in vivo as assessed by plasma VL. Including the initial anti-gp41 immunoglobulin G (IgG), IgM, or both responses in the model did not significantly impact the early dynamics of plasma VL. These results demonstrate that the first IgM and IgG antibodies induced by transmitted HIV-1 are capable of binding virions but have little impact on acute-phase viremia at the timing and magnitude that they occur in natural infection.     

Genetics of resistance to the African trypanosomes. III. Variant-specific antibody responses of H-2-compatible resistant and susceptible mice

Genetically based differences in variant-specific immunity to the African trypanosomes were examined. H-2-compatible inbred mouse strains that differed in relative resistance were infected with Trypanosoma rhodesiense clone LouTat 1. Antibody responses to exposed epitopes of the LouTat 1 variant-specific surface glycoprotein (VSG) were measured. Relatively resistant B10.BR mice (H-2k) made predictable IgM antibody responses to the VSG of LouTat 1 which were associated with clearance of the LouTat 1 variant antigenic type from blood; IgG responses to LouTat 1 surface antigen appeared after clearance occurred, and were lower than peak titers of IgM. Intermediately susceptible CBA mice (H-2k) also made predictable IgM and IgG responses which followed the same pattern as the more resistant strain. Peak titers were lower for both Ig classes, however, and a delayed appearance of antibody was correlated with delayed clearance of LouTat 1. In contrast to B10.BR and CBA mice, the susceptible C3H mice (H-2k) failed to make detectable antibodies to LouTat 1 surface antigen and also failed to control the first peak of parasitemia. The absence of immunity in infected C3H mice was selective for antibody to exposed epitopes of LouTat 1 VSG because antibody was detectable to invariant VSG or internal trypanosome antigens. Also, the C3H strain was shown not to be a genetic nonresponder to LouTat 1 surface antigen because VSG-specific antibodies appeared within 1 wk after trypanocidal chemotherapy. Finally, we demonstrated that the susceptibility of C3H mice was not associated with an inability of the mononuclear phagocyte system to clear the parasites because drug cure, passive transfer of immune serum, or sensitization of trypanosomes with antibody all led to trypanosome clearance from blood by the liver. In summary, we show for the first time that major differences in variant-specific immunity occur in MHC-compatible animals after infection with the African trypanosomes.     

Ontogeny of the immune system of fish

Information on the ontogeny of the fish immune system is largely restricted to a few species of teleosts (e.g., rainbow trout, catfish, zebrafish, sea bass) and has previously focused on morphological features. However, basic questions including the identification of the first lympho-hematopoietic sites, the origin of T- and B-lymphocytes and the acquisition of full immunological capacities remain to be resolved. We review these three main topics with special emphasis on recent results obtained from the zebrafish, a new experimental model particularly suitable for study of the ontogeny of the immune system because of its rapid development and easy manipulation. This species also provides an easy way of creating mutations that can be detected by various types of screens. In some teleosts (i.e., angelfish) the first blood cells are formed in the yolk sac. In others, such as zebrafish, the first hematopoietic site is an intraembryonic locus, the intermediate cell mass (ICM), whereas in both killifish and rainbow trout the first blood cells appear for a short time in the yolk sac but later the ICM becomes the main hematopoietic area. Erythrocytes and macrophages are the first blood cells to be identified in zebrafish embryos. They occur in the ICM, the duct of Cuvier and the peripheral circulation. Between 24 and 30 hour post-fertilization (hpf) at a temperature of 28 degrees C a few myeloblasts and myelocytes appear between the yolk sac and the body walls, and the ventral region of the tail of 1-2 day-old zebrafish also contains developing blood cells. The thymus, kidney and spleen are the major lymphoid organs of teleosts. The thymus is the first organ to become lymphoid, although earlier the kidney can contain hematopoietic precursors but not lymphocytes. In freshwater, but not in marine, teleosts the spleen is the last organ to acquire that condition. We and other authors have demonstrated an early expression of Rag-1 in the zebrafish thymus that correlates well with the morphological identification of lymphoid cells. On the other hand, the origins and time of appearance of B lymphocytes in teleosts are a matter of discussion and recent results are summarized here. The functioning rather than the mere morphological evidence of lymphocytes determines when the full immunocompetence in fish is attained. Information on the histogenesis of fish lymphoid organs can also be obtained by analysing zebrafish mutants with defects in the development of immune progenitors and/or in the maturation of non-lymphoid stromal elements of the lymphoid organs. The main characteristics of some of these mutants will also be described.     

Autogenous Immunity to Endogenous RNA Tumor Virus: Differential Reactivities of Immunoglobulins M and G to Virus Envelope Antigens

The autogenous humoral immune response of mice to their endogenous leukemia virus has been examined in terms of the reactivities of individual classes of antibody present in normal B6C3F(1) serum. Whole serum and the immunoglobulin (Ig) M and IgG fractions of serum from animals of different age groups were compared by radioimmune precipitation assays and viral infectivity neutralization assays. Both IgM and IgG fractions were able to precipitate virus, although not as effectively as whole serum. Virus-specific antibody levels, as well as total antibody concentrations in whole serum, appeared to increase with age. Sodium dodecyl sulfate gel electrophoresis analysis was performed with immune precipitates obtained when whole serum or 19 or 7S fractions from animals of different age groups were reacted with disrupted virus. The 19S antibody fraction reacted with three antigenic determinants on the viral envelope. These antigens have apparent molecular weights of 17,000, 43,000, and 68,000. The last two appear to be glycoproteins and may correspond to the M(2) and M(1) antigens. In contrast, the 7S component reacted only with the 17,000-molecular-weight protein. Neutralization assays against BALB:virus-2, a xenotropic endogenous mouse type C virus, revealed that 19S and whole serum but not the 7S fraction possessed neutralizing activity. These findings indicate that there are differential reactivities of IgM and IgG antibodies in normal serum of B6C3F(1) mice, with respect to both recognition of viral envelope antigens and neutralization of endogenous MuLV. These results are consistent with the hypothesis that the autogenous humoral immune response is a systemic host function that may be important in the regulation of endogenous type C virus expression in vivo.     

Comparative study of some haematological and biochemical blood parameters in fishes from the Skagerrak

Marine fishes caught in the Skagerrak, 27 different species representing various groups of fishes (Cyclostomi, Holocephali, Elasmobranchii and Teleostei), were examined for the following haematological and biochemical blood parameters: haematocrit, haemoglobin, mean corpuscle haemoglobin concentration, total plasma protein, blood glucose and blood lactate. Interspecies variations as well as variations within some species were observed. The haemoglobin values for all species showed a positive correlation to the corresponding haematocrit values. Relatively low values for haematocrit and haemoglobin were found in cyclostomes, holocephaleans and elasmobranchii compared to the majority of teleosts. Within the teleost group, the haematocrit and haemoglobin levels were positively correlated with the activity of the fish species. The cyclostome Myxine glutimsa L. had a total plasma protein content in the same range as most teleosts, whereas holocephaleans, elasmobranchii and the deep-water teleost Coryphaenoides rupestris Gunnerus showed comparatively low values. Among teleosts some relationship seemed to exist between the total plasma protein level and the activity of the fish species. In addition, a correlation between plasma protein content and levels of blood lipids were noted. Values for blood glucose and blood lactate were found to be lower in cyclostomes, holocephaleans and elasmobranchii than in most teleosts. Higher blood glucose levels were observed in the more active teleost species.     

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.     

Antigenic analysis of grass carp reovirus using single-chain variable fragment antibody against IgM from Ctenopharyngodon idella

Grass carp (Ctenopharyngodon idella) is an important species of freshwater aquaculture fish in China. However, grass carp reovirus (GCRV) can cause fatal hemorrhagic disease in yearling populations. Until now, a strategy to define the antigenic capacity of the virus’s structural proteins for preparing an effective vaccine has not been available. In this study, some single-chain variable fragment antibodies (scFv), which could specifically recognize grass carp IgM, were selected from a constructed mouse naïve antibody phage display cDNA library. The identified scFv C1B3 clone was shown to possess relatively higher specific binding activity to grass carp IgM. Furthermore, ELISA analysis indicated that the IgM level in serum from virus-infected grass carp was more than two times higher than that of the control group at 5–7 days post infection. Moreover, Western blot analysis demonstrated that the outer capsid protein VP7 has a specific immuno-binding-reaction with the serum IgM from virus-infected grass carp. Our results suggest that VP7 can induce a stronger immune response in grass carp than the other GCRV structural proteins, which implies that VP7 protein could be used as a preferred immunogen for vaccine design.     

The venous circulation: a piscine perspective.

Vascular capacitance describes the pressure-volume relationship of the circulatory system. The venous vasculature, which is the main capacitive region in the circulation, is actively controlled by various neurohumoral systems. In terrestrial animals, vascular capacitance control is crucial to prevent orthostatic blood pooling in dependent limbs, while in aquatic animals like fish, the effects of gravity are cancelled out by hydrostatic forces making orthostatic blood pooling an unlikely concern for these animals. Nevertheless, changes in venous capacitance have important implications on cardiovascular homeostasis in fish since it affects venous return and cardiac filling pressure (i.e. central venous blood pressure), which in turn may affect cardiac output. The mean circulatory filling pressure is used to estimate vascular capacitance. In unanaesthetized animals, it is measured as the central venous plateau pressure during a transient stoppage of cardiac output. So far, most studies of venous function in fish have addressed the situation in teleosts (notably the rainbow trout, Oncorhynchus mykiss), while any information on elasmobranchs, cyclostomes and air-breathing fishes is more limited. This review describes venous haemodynamic concepts and neurohumoral control systems in fish. Particular emphasis is placed on venous responses to natural cardiovascular challenges such as exercise, environmental hypoxia and temperature changes.     

Toxoplasma gondii: cold stress-induced modulation of antibody responses.

Physical or psychological stressors have been shown to have significant consequences in the immune function and the outcome of disease in human and animal models. Recent work has demonstrated that products released during stress, such as glucocorticoids and catecholamines, can profoundly influence the in vitro growth of pathogens by modulating immune responses. The present study examined the effects of a physical stressor (cold stress) on antigens of Toxoplasma gondii that elicits an antibody-mediated immune response during the acute and chronic phases of infection. Sera obtained from different groups of mice subjected to cold stress during the acute and chronic phases of T. gondii infection were used to measure the levels of antibodies and to localize by Western blot the dominant antigens eliciting IgG and IgM antibody responses. Serum antibodies collected from stressed and infected mice recognized antigens different from those recognized by infected mice without stress. During the acute phase, a stronger IgM antibody response against antigens of 30, 42, 54, and 60 kDa was detected in stressed animals at 3 weeks postinfection. In addition, a 5-kDa antigen was specifically detected in mice subjected to stress during the acute and chronic phases of infection. Levels of specific IgG were increased in infected and in infected and stressed animals that underwent stress in the chronic phase. IgM production did not increase following cold stress in the chronic phase. These results suggest that the strong antibody response in stressed animals is associated with longer parasite persistence in circulation. Stress modulated not only the host immune response but also the ability of parasite antigens to elicit specific antibody responses by the host.     

Potentiation of antibody responses by specific IgM: specificity and thymus dependency

Modulation of antibody responses induced by IgM directed against the immunogen was investigated. When IgM directed against ox erythrocytes (ORBC) was given together with trinitrophenyl (TNP)-ORBC, the subsequent antibody response to the carrier, ORBC, as well as the response to the hapten, TNP, was potentiated. In contrast, IgG with carrier specificity inhibited both responses. The hapten-specific potentiation was found in both direct and indirect plaques, and was antigen-dose dependent, i.e., no potentiation was found with the lowest antigen doses. The response to 2,4-dinitrophenyl (DNP)-labeled proteins was potentiated by a monoclonal IgM with specificity for the hapten. The effects were observed both in primary and secondary responses. One strict requirement for IgM potentiation to occur was observed. The determinant against which potentiation was achieved had to be physically linked to the determinant against which the IgM was directed, be it hapten or carrier determinants. Thus, irrelevant IgM-antigen complexes were incapable of potentiating the responses. Similar specificity requirements were found for IgG induced suppression of antibody responses. Experiments with nude mice and their euthymic littermates showed that IgM potentiation of antibody production is T-cell dependent. Furthermore, passive transfer of carrier-primed spleen cells together with antigen challenge suggests that IgM potentiation of secondary antibody responses is dependent on specific carrier-primed immune T cells.     

The venous circulation: a piscine perspective

Vascular capacitance describes the pressure–volume relationship of the circulatory system. The venous vasculature, which is the main capacitive region in the circulation, is actively controlled by various neurohumoral systems. In terrestrial animals, vascular capacitance control is crucial to prevent orthostatic blood pooling in dependent limbs, while in aquatic animals like fish, the effects of gravity are cancelled out by hydrostatic forces making orthostatic blood pooling an unlikely concern for these animals. Nevertheless, changes in venous capacitance have important implications on cardiovascular homeostasis in fish since it affects venous return and cardiac filling pressure (i.e. central venous blood pressure), which in turn may affect cardiac output. The mean circulatory filling pressure is used to estimate vascular capacitance. In unanaesthetized animals, it is measured as the central venous plateau pressure during a transient stoppage of cardiac output. So far, most studies of venous function in fish have addressed the situation in teleosts (notably the rainbow trout, Oncorhynchus mykiss), while any information on elasmobranchs, cyclostomes and air-breathing fishes is more limited. This review describes venous haemodynamic concepts and neurohumoral control systems in fish. Particular emphasis is placed on venous responses to natural cardiovascular challenges such as exercise, environmental hypoxia and temperature changes.     

Serodiagnostics of chlamydial infections--significance of positivity in IgA and/or IgM antibody classes only

There was followed the development of serological findings in patients with proved positivity only in classes IgA and/or IgM of chlamydial antibodies (without IgG), which can be suspected of showing "false" positivity. 184 patients were repeatedly examined for chlamydial antibodies in their sera (interval between collections up to three months) using a genus specific rELISA. Sera were also tested for the evidence of IgM antibodies against capside antigen of Epstein-Barr virus (EBV) and against cytomegalovirus (CMV) using ELISA methods. In 75 (40.8%) of patients, IgA/IgM individual positivities were demonstrated even during the following sample test(s). In 28 (15.2%) of them, IgG evidence preceded and in 29 (15.7%) other patients positive seroconversion followed in this class. In 13 (7.1%) patients, IgG antibodies disappeared and subsequently reappeared. Only in 39 (21.2%) of these probands, antibodies IgA/IgM were not demonstrated at another examination. Active EBV, resp. CMV infection was proved in 24 (13.0%), resp. in 18 (9.8%) of patients. It is concluded that the evidence of positivities only in classes IgA and/or IgM mostly signal the onset of a primary infection (reinfection) or an active infection in patients with IgG production failures respectively. In these cases, a "false" positivity can be supposed to occur only in a minor extent.     

Murine immune response to the Neisseria meningitidis group C capsular polysaccharide. I. Ontogeny

The immune response to polysaccharide Ag develops late in ontogeny and the underlying mechanisms of the infant unresponsiveness are poorly understood. The development of vaccines that will prove efficacious in infants has been hindered by the lack of animal systems suitable for studying immunity to human pathogens. We have examined the BALB/c murine response to the capsular polysaccharide of Neisseria meningitidis group C (MCPS), a homopolymer of alpha(2----9) sialic acid, as a model system for the development of immunity to bacterial polysaccharides in man. We have observed the appearance of natural antibody of both IgM and IgG classes which increases with age, and the transfer of maternal IgG to the offspring. Both the naturally occurring and postimmunization serum responses are restricted to the IgM and IgG3 isotypes, and include antibody titers to both MCPS as well as a natural O-acetyl-negative variant (OAc-). The preimmune anti-OAc- antibodies, in contrast to anti-MCPS, were restricted to the IgM class, whereas after immunization with MCPS both IgM and low titers of IgG3 antibodies to OAc- were produced. These studies demonstrate that the BALB/c mouse strain shows a markedly similar immune profile to that observed in man.