Expression of Ia antigens on hapten-specific B cells. I. Delineation of B-cell subpopulations

Altered immune response patterns in murine syngeneic pregnancy: presence of natural null suppressor cells in maternal spleen identifiable by monoclonal antibodies

Expression of certain autologous lymphocyte-activating antigenic determinants on the developing embryo is known to provide a stimulus for maternal anti-fetal autoproliferative responses. If left unregulated these responses could exert negative influences on the reproductive process by converting to autoaggressive forms of immune reactivity. In normal circumstances, immunological reactions of this nature are therefore likely to be under the control of pregnancy-associated immunoregulatory elements found within the maternal/fetal environment. In the present investigation we describe a naturally occurring splenic inhibitory cell type devoid of conventional T, B, and macrophage surface markers associated with syngeneic murine pregnancy that is capable of exerting potent immunosuppressive effects on an in vitro expression of fetal/newborn T cell autoreactivity, namely the autologous mixed lymphocyte reaction (AMLR). Maternal spleen cells inhibitory for AMLR were found to be highly resistant to cytotoxic pretreatment with a panel of conventional antisera directed against T cell-specific antigenic determinants. The non-T nature of the natural splenic suppressor cell was further indicated by experiments showing that purified spleen T cells had no inhibitory activity. Pregnancy spleen cell populations that were effectively depleted of macrophages retained full ability to inhibit AMLR. Maternal suppressor activity could be localized to the spleen cell population bearing receptors for the B cell-specific lectin, soybean agglutinin (SBA). A panel of monoclonal antibodies prepared against enriched populations of suppressor cells was screened and selected for specific reactivity using an ELISA against glutaraldehyde-fixed SBA+ spleen cell subpopulations from pregnant versus virgin animals. Several of the monoclonals developed against suppressor-enriched spleen cell populations from isopregnant as well as allopregnant animals were effective in reducing or eliminating suppressor cell activity following cytotoxic pretreatment in the presence of complement. The novel set of anti-suppressor monoclonal antibodies described here should prove useful in furthering the isolation and characterization of pregnancy-associated suppressor cells and in determining their relationship to natural suppressor cell populations described in other systems.

Ontogenical studies on kinetics of lipopolysaccharide-induced response to bromelain-treated mouse erythrocytes in mouse spleen cells. II. Response of spleen cells with or without Fc receptors, C3 receptors, or Ia antigens

Lipopolysaccharide (LPS)-induced plaque-forming cells secreting IgM (IgM-PFC) and antibodies against bromelain-treated mouse erythrocytes (anti-BrMRBC PFC) on Days 1 and 2 of cultures were quantitatively estimated in spleen cells from mice of various ages. The concentrations of the four groups of PFC changed independently with age. The LPS dose dependency of the PFC response was markedly different between PFC on Days 1 and 2, but not different between anti-BrMRBC PFC and IgM-PFC or between 2- and 10-week-old mice. In a second experiment, spleen cells from 2- and 10-week-old mice were separated into subpopulations with or without Fc receptors, C3 receptors, or Ia antigens, and the LPS-induced PFC responses were quantitatively assessed in each subpopulation. Both the receptor-bearing and -lacking populations included LPS-reactive B cells, and the percentages of the LPS-reactive B cells recovered in the receptor-bearing population increased with age. However, the percentages of anti-BrMRBC PFC recovered in each receptor-bearing or -lacking population were different from those of IgM-PFC. In Ia- populations, the percentages of IgM-PFC on Day 2 were obviously higher than those on Day 1, and both of the percentages increased with age. These results suggest that the four groups of LPS-reactive B cells can be discriminated from each other by their LPS dose dependency and their cell surface markers, and that they develop differently during ontogeny.

Primary in vitro plaque-forming cell response to DAGG-Ficoll: LPS-induced enhancement mediated by interleukin-1

The specific primary in vitro plaque-forming cell (PFC) response of C57B1/6 nu/nu spleen cells to the Type 2 T-independent (TI-2) antigen DAGG-Ficoll was analysed in the absence of T cell help in a serum-free medium. Lipopolysaccharide (LPS) enhancement of the antigen-specific response was shown to be mediated by soluble factors contained in the supernatants of LPS-induced bone marrow-derived macrophages. The activity of these supernatants was not associated with residual LPS, since the antigen-specific response of B cells from mice genetically deficient in LPS receptors was equally well enhanced. The activity of these supernatants was associated with interleukin-1 (IL-1) and partially purified IL-1 prepared from P388D1 cells also enhanced the primary in vitro response to DAGG-Ficoll. Limiting dilution analysis experiments showed that only in the presence of exogenously added IL-1 could a single cell type, presumably the B cells, be shown to be limiting.

Subpopulations of lymphocytes in maternal peripheral blood during pregnancy

The fetus can be considered an allograft with up to one-half of its MHC antigens being potentially recognized by the mother as foreign. This study compares expression of OKT3, OKT4, OKT8, Kappa, Lambda and Ia antigens on lymphocytes in the peripheral blood of normal non-pregnant women, normal pregnant women, patients who are chronic spontaneous aborters and pregnant insulin-dependent diabetic women. Monoclonal antibodies and cytofluorometric analyses were used for these determinations. There were no significant differences (P = 0.01) between these groups for T-cell markers. A statistically significant (P = 0.001) increased ratio of cells bearing surface immunoglobulin to those expressing Ia antigen (K&L/Ia) was observed between normal non-pregnant controls and women with a history of chronic spontaneous abortion. It is concluded that T-lymphocytes in the peripheral blood do not demonstrate a phenotypic abnormality that would account for the non-rejection of the fetal allograft; however, women with chronic spontaneous abortion may have abnormal B-cell differentiation or T-cell activation that mediates chronic spontaneous abortion.

Characterization of T-cell-soluble factors modulating the expression of Ia and H-2 antigens on BALB/c B lymphoma cell lines

The effect of supernatants of concanavalin A-activated spleen cells (CAS) on the expression of various antigens, especially Ia antigens, on BALB/c B lymphoid cells, was examined. This study demonstrates the following: (i) CAS enhanced the expression of Ia antigens on four out of five BALB/c lymphoid cell lines. (ii) CAS selectively modulates the expression of Ia and H-2D, but not sIgM or viral gp70 expression, on X16C 8.5 tumor cells. The enhanced levels of Ia expression on B lymphoid tumor cells were also detected by using anti-Ia monoclonal antibodies. (iii) The molecular weight of soluble factor(s) affecting Ia and H-2 was approximately 40,000 estimated by gel filtration on a Sephadex G-200 column. (iv) Type 1 interferon but not interleukin 1, interleukin 2, or T-cell-replacing factor enhanced the expressions of Ia and H-2D antigens. (v) The activity of CAS-modulating Ia and H-2 antigens was eliminated by acidic treatment. It was concluded from this study that at least one of the factor(s) in CAS, modulating the antigenic expression of B-lymphoid cells, was interferon-like in nature. From our findings, a possible immunoregulatory mechanism by interferon was suggested: T cells, after stimulation of mitogens or antigens, secrete interferons which modulate the expression of Ia and H-2 on B cells. Then B cells, whose Ia and H-2 were modulated selectively by T-soluble factors(s), might interact with T cells much more efficiently.

Modulation of B and T cell subsets in mice treated with fractionated total lymphoid irradiation. II. Tolerance susceptibility of B cell subsets

Total lymphoid irradiation (TLI) results in long-lasting changes in the characteristics of both T and B cells suggestive of arrested maturation. A characteristic feature of immature B cells is their high susceptibility to tolerance induction. This study examines the susceptibility to tolerance to bovine serum albumin (BSA) of TLI-treated mice. Two experimental protocols were designed. In the first, tolerance to BSA was induced in TLI-treated adult (BALB/c X C57BL/6)F1 mice, and the ability of B cells of those mice to respond to BSA was assessed in an adoptive transfer system. In the second experimental protocol, tolerance was induced in adoptive hosts reconstituted with purified B cells originating from TLI-treated mice and with splenic T cells of normal, untreated mice. Results obtained in these two systems clearly demonstrated that splenic B cells of TLI-treated mice are highly susceptible to tolerance induction. This high susceptibility of B cells is linked neither with the elevation of immature T cells nor with induced T suppressor cells which arise due to the long-term malfunction of the thymus. Tolerance could be induced in TLI cells even 4 months after termination of the treatment. Thus, maturation processes of B cells in TLI-treated mice are arrested for long periods of time.

Patterns of isotype expression by B cell clones responding to thymus-dependent and thymus-independent antigens in vitro

It was found that the Type 2 thymus-independent (TI-2) antigens bacterial levan, trinitrophenyl-Ficoll, and pneumococcal carbohydrate vaccine (PnC) stimulate clonal expansion and antibody secretion in splenic fragments from either hemocyaninprimed or unprimed irradiated recipients bearing B cells from unprimed donors. The in vitro stimulation with TI-2 antigens leads to the expression of isotype switching and provides a more balanced variety of isotypes than is usually observed in vivo. Still, some characteristic patterns of isotypes expressed in vivo to either TI-2 or thymus-dependent (TD) antigens are preserved in vitro. Frequencies of phosphocholine (PC)-reactive B cells responding to either PnC or to PC-hemocyanin (PC-Hy) suggest an appreciable overlap in populations responding to these TI and TD forms of antigen. The existence of a population responsive to either form of PC determinant is supported by the observation that many clones arising in the presence of both forms of antigen express patterns of isotypes that appear as summations of those distinct patterns shown by clones responding to only one form or the other. These data suggest that PC-Hy- and PnC-responding cells may derive from a linear rather than a branched pathway of B cell development and that expression of isotype switching over the lifetime of a developing B cell clone may be regulated in a manner dependent on the form of the stimulating antigen.

Genetic regulation of antibody response to sheep red blood cells: linkage to H-2 complex

The analysis of anti-sheep red blood cell (SRBC) antibody production in the congenic resistant (CR) mouse strains A/J and A.BY, B10 and B10.A showed that the level of IgG antibodies after immunization with SRBC is controlled by a gene(s) localized in the H-2 complex. The use of h2,h4, and i5H-2 recombinant haplotypes allowed us to map this gene into a region proximally defined by subregion I-J and distally by region H-2G. The IgG antibody level is simultaneously under the influence of non-H-2 genes, of which those of the A/J strain origin determine the high IgG level and those of the B10 strain origin determine the low IgG level. Weights of spleens of the A/J and B10 mice before and after primary and secondary immunization with SRBC were compared. Before immunization, spleens of the A/J mice were approximately 10% lighter than those of the B10 mice (0.08 g vs 0.09 g). After the first immunization the spleen weights equalized (0=0.12 g). Four days after the second immunization the weight of the spleens of A/J mice increased by 312% (relative to the nonimmune state) while that of the B10 mice increased by only 74%. These findings indicate that the cell antigen-specific proliferation in spleens of A/J mice was considerably higher than that in spleens of B10 mice.

Functional analysis of T cells expressing Ia antigens. I. Demonstration of helper T-cell heterogeneity

We have examined the expression of I-region antigens on functional subpopulations of murine T cells. A.TH anti-A.TL (anti-Ik, Sk, Gk) alloantiserum was raised by immunization of recipients with concanavalin A (Con A) stimulated thymic and peripheral T-cell blasts. In contrast to similar antisera made by conventional methods, the anti-Ia blast serum was highly cytotoxic for purified T lymphocytes. Moreover, it reacted in a specific fashion with T cells having particular functions. Treatment of keyhole limpet hemocyanin (KLH)-primed B10.A (H-2 alpha) T cells with this antiserum plus complement resulted in the elimination of helper activity for B-cell responses to trinitrophenyl-KLH. Inhibition was shown to be a result of the selective killing of one type of helper T cell whose activity could be replaced by a factor(s) found in the supernate of Con A-activated spleen cells. A second type of helper cell required for responses to protein-bound antigens appeared to be Ia-. By absorption and analysis on H-2 recombinants, at least two specificities were detectable on helper T cells; one mapping in the I-A subregion and a second in a region(s) to the right of I-J. In addition, the helper T cell(s) involved in the generation of alloreactive cytotoxic lymphocytes was shown to be Ia+, whereas cytotoxic effector cells and their precursors were Ia- with this antiserum. These results provide strong evidence for the selective expression of I-region determinants on T-cell subsets and suggest that T-cell-associated Ia antigens may play an important role in T-lymphocyte function.

Organization of the immune response genes

The I region of the major histocompatibility complex contains immune response genes that display considerable polymorphism; that is, there are many alleles at each locus. These genes regulate the immune response to antigen by mediating intercellular communication among lymphoreticular cells. An analysis of the primary structure of the products of two subregions of (I-A, I-E/C) was undertaken in order to understand the genetic organization of the region, the evolution of the genes and, eventually, their function.

Hyper immunoglobulin M immunodeficiency. (Dysgammaglobulinemia). Presence of immunoglobulin M-secreting plasmacytoid cells in peripheral blood and failure of immunoglobulin M-immunoglobulin G switch in B-cell differentiation

The peripheral blood lymphocytes of nine patients with hyper immunoglobulin (Ig)M immunodeficiency were studied in an attempt to define the cellular basis of this disorder. B cells were normal in number but qualitatively abnormal in all patients. Approximately one-half of the B cell consisted of small lymphocytes (7-9 mum in diameter) bearing surface IgM and IgD, as well as C3 receptors. These cells were driven to secrete IgM but not IgG after in vitro stimulation by pokeweed mitogen. In the blood there were also large lymphocytes (10-14 mum in diameter) that possessed surface as well as intracytoplasmic IgM but lacked C3 receptors. These cells spontaneously secreted large amounts of IgM in vitro and on electron microscopy were found to be rich in rough endoplasmic reticulum. Such a subpopulation of lymphoid cells was not detected in normal peripheral blood and was unique for all patients with hyper IgM immunodeficiency studied.T cells from all patients were normal in number and in function both in vivo and in vitro and were able to generate adequate T-cell help to support IgG synthesis by normal B cells. No evidence was obtained for T cells capable of suppressing normal IgG synthesis in any of the patients after coculture with normal peripheral blood lymphocytes. The defect in hyper IgM immunodeficiency is intrinsic to B cells, which fail to switch from IgM to IgG synthesis.

Separation of T and B lymphocytes from various mouse strains by density gradient electrophoresis

T and B mouse spleen lymphocytes were separated by density gradient electrophoresis on the basis of their surface charge. In all strains examined, the T lymphocytes were found in the high mobility fractions and the B in the low. The T and B cells were separated completely in most fractions, with some overlapping in the middle. Significant differences were found in the electrophoretic distribution profiles between the strains: C57BL/6j, C57BL/10j, (BALB/cXC57BL/6j)F1, and all the following: B6.C-H-2d/cBy (congenic to C57BL/6j), BALB/c, CBA/H/T6j, C57BL/10Sn, and C3H. The C57BL/6j and the (BALB/cXC57BL/6j)F1 cells appear more heterogeneous as far as electrophoretic mobility is concerned. Almost all the other strains give two major peaks. Moreover, the high mobility areas are less populated in the C57BL/6j and the (BALB/cXC57BL/6j)F1 animals than in all the others. The above differences were found consistently when cells prepared by different methods were electrophoresed. It is concluded that the surface charge of lymphocytes may be genetically determined. Possible dependency on the H-2 complex or non-H-2 areas is discussed.

Immunopotentiation and anabolism induced by sodium diethyldithiocarbamate

Sodium diethyldithiocarbamate, DTC, enhances over a large range of doses macrophage listericidal capacity and T cell activities in terms of increased IgG-antibody forming spleen cells and delayed hypersensitivity levels. Such immunopotentiation is not associated with splenomegalia or increase in lymphocyte counts. Immunopotentiation requires a preexisting link between carbon disulfide and diethylamine, since both moieties were inactive if administered alone or on separate body sites. DTC demonstrates also an anabolic effect on mice emanciated by administering a B. melitensis cell-wall fraction. The role of DTC on hormonal production is discussed in relation to hormone-mediated action on T cell induction.

Phytohemagglutinin stimulation of enhanced immunoglobulin G production in mice inoculated with type III pneumococcal polysaccharide

In BALB/c mice previously inoculated intraperitoneally with an immunogenic dose of the T-independent antigen type III pneumococcal polysaccharide, the intravenous administration of the T-cell activating agent phytohemagglutinin P causes a pronounced increase in the number and relative proportion of immunoglobulin G-producing cells. These results, detected by a modified hemolytic plaque assay, were supported by finding increased levels of serum immunoglobulin G anti-type III pneumococcal polysaccharide in the treated mice. A comparable stimulation of immunoglobulin G antibody-producing cells was not induced in phytohemagglutinin P-treated nude mice, indicating that the change in class of the predominant antibody is attributable to the activation by the phytohemagglutinin P of a T-cell population. Under the conditions of these experiments, phytohemagglutinin P also promotes a progressive suppression of the antibody-forming cells during the response to type III pneumococcal polysaccharide.

Functional characterization of lanatoside-C-responsive cells

Lanatoside C, a digitalis glycoside previously shown to be a polyclonal B-cell activator (PBA), was found to stimulate immature cells residing in fetal liver, bone marrow and spleen but also to activate cells from peripheral lymph nodes and peripheral blood. The proliferative response obtained in spleen cells was not affected by macrophage removal, whereas anti-Ig or anti-Ia antiserum pretreatment partially inhibited the responses. Removal of T cells by a pretreatment with anti-Thy 1.2 antiserum plus complement caused a marked increase in the proliferative response of the remaining cells, suggesting the existence of a naturally occurring suppressor T cell for glycoside-induced mitogenesis. Synergy experiments with 'classical' PBAs and lanatoside C, given simultaneously or subsequently, suggest an overlap between the lanatoside-C-responding cell population and the dextran sulphate (DxS)- and lipopolysaccharide (LPS)-sensitive cells. Since DxS-induced activation of B cells is dependent on macrophages, it is suggested that lanatoside C may be used as a functional marker for direct activation of immature B cells.

In vitro analysis of allogeneic lymphocyte interaction. II. I-region control of the activity of a B-cell-derived H-2-restricted allogeneic effect factor and its receptor during B-cell activation

A genetically restricted allogeneic effect factor (AEF) derived from a mixed lymphocyte culture reaction between Ia-negative activated responder cells and irradiated T-cell-depleted stimulator cells was characterized. Restricted AEF is a B-cell-derived soluble helper factor which consists in part of Ia antigens controlled by the I-A subregion of the stimulator haplotype; additional control by the I-B, I-E, and I-C subregions, although unlikely, could not be excluded. This factor helps B cells of only its own haplotype or of haplotypes which carry an I-A and/or I-B subregion identity. Unprimed as well as hapten-primed Ia-positive B cells express a receptor for restricted AEF. The results indicate that the B-cell receptor for AEF is determined by the I-A subregion. Both restricted AEF and its receptor may therefore be products of the same I-region gene(s). The data are compatible with the hypothesis that the AEF Ia antigens serve as a second signal required for B-cell activation to IgG antibody production.

Structure of murine Ia antigens: partial NH2-terminal amino acid sequences of products of the I-E or I-C subregion

Partial amino acid sequence of the Ia molecule encoded by the I-E or I-C (I-EC) subregion of the major histocompatibility complex of the mouse are presented. The Ia molecule appears to be comprised of two noncovalently associated polypeptides. The larger subunit, alpha, has an approximate molecular weight of 35,000 and the smaller subunit, beta, an approximate molecular weight of 28,000. Several interesting homology relationships (or the lack thereof) are apparent when the Ia polypeptides from the I-EC subregion are compared both with their counterparts from man and guinea pig and with the molecules encoded in the I-A subregion. Clearly the most impressive homology relationship is that seen between the alpha polypeptide from the I-EC subregion of mouse and its human counterpart. This is in striking contrast to the beta polypeptide, which bears no apparent homology to its human counterpart.

In vitro analysis of allogeneic lymphocyte interaction. I. Characterization and cellular origin of an Ia-positive helper factor-allogeneic effect factor

A soluble allogeneic effect factor (AEF) was produced by using H-2 congenic mouse strains and a serum.free cell culture medium. An AEF derived from untreated activated responder cells and irradiated stimulator cells provided helper cell function in a primary and secondary antibody response for both T-cell-depleted responder B cells and stimulator B cells. This interaction may be determined by genes situated in the I-A and I-B regions: additional K-region control was not excluded. Ia antigens, but neither H-2 nor Ig determinants are molecular constituents of AEF. The active components of this AEF consist, in part, of Ia antigens derived from both the activated responder cell population and irradiated stimulator cell population. An AEF derived from Ia negative responder cells and irradiated T-cell- depleted stimulator cells helps a secondary antibody response of T-cell- depleted stimulator B cells but not responder B cells. This genetically restricted AEF contains Ia antigens determined by the stimulator haplotype but not the responder haplotype. The priming antigen, DNP- keyhole limpet hemocyanin, is not a component of restricted AEF. The data suggest that restricted AEF may be a product of a stimulator B cell and/or macrophage. They support the hypothesis that the recognition by allogeneic T cells of Ia antigens on B cells activates the B cell to IgG antibody production.

Influence of splenectomy on first set rejection reactions of C57BL/6 females to male skin isografts

The tempo with which C57BL/6 females reject male skin isografts is determined in part by the immunogenicity of the H-Y antigen and in part by the capacity of the host to respond immunologically. Our studies indicate that the spleen plays an important role in determining the briskness of the rejection process in that splenectomy 7-30 days before grafting with male skin significantly shortens the survival time. The results of reconstitution experiments suggests that a population of cells is present in spleens of normal, but not specifically sensitized, females which can restore the conventional first set reaction in splenectomized females. It is inferred that this resident population normally operates in spleen-intact females to delay the development of specific effector responses. Lymphoid cells from H-Y antigen-sensitized, splenectomized females failed to evoke graft-vs.-host responses in males whereas similar cells from females with spleens intact did possess graft-vs.-host potential.

Characterization of a carcinogen-induced murine B lymphocyte cell line of C3H/eB origin

A carcinogen-induced lymphoid tumor, denoted 38C-13, obtained in a T cell-depleted mouse of C3H/eB strain, was adapted to continuous culture in vitro and characterized with respect to its cell surface components. The cells possess IgM class immunoglobulins on their surface but do not secrete it. This membrane IgM is composed of mu and L-chains that are similar in apparent molecular weight to those of an IgM myeloma protein. It is also homogeneous as revealed by isoelectric focusing. The cells possess Fc receptors but lack complement receptors as well as Thy-1 and Ia alloantigens. These characteristics indicate that 38C-13 cells are transformed counterparts of small B lymphocytes at an early stage of differentiation.

Ontogeny of Ia and IgD on IgM-bearing B lymphocytes in mice

We used immunofluorescence to examine the developmental relationship of Ia and IgD on B cells. Pre-B cells in fetal liver did not express Ia. Only very few surface IgM-positive (sIgM+) B cells in fetal spleen were found to be Ia+ and were weakly stained for Ia. After birth there was a linear increase in the proportion of sIgM+ spleen cells which expressed Ia, reaching 95% by 9 days. Adult bone marrow also contains a sizeable proportion of sIgM+ Ia- cells. Unstimulated cells from fetal or newborn liver and spleen expressed Ia at the same rate in culture. Anti-Ia antisera suppressed the LPS-induced differentiation of IgM and IgG plasma cells in cultures of neonatal lymphocytes. Ia was also detected on IgM and IgG plasma cells in vitro suggesting that lipopolysaccharide (LPS)-stimulated B cells by may express Ia antigens, induced by LPS, or appearing as part of normal differentiation. IgD did not appear on sIgM+ cells until 3 days of age and then rose slowly to reach adult levels later than Ia antigens.

Functional specificity of thymus- dependent lymphocytes

Thymus-dependent lymphocytes display a restricted range of specificity when compared to thymus-independent (B) lymphocytes. They react particularly to thymus-dependent protein and cell surface antigens and to products of genes encoded in the MHC. In addition, T lymphocytes have important functions in regulating the immune response and in discriminating of self from nonself. Recent work indicates that individual T lymphocytes possess receptors that interact with both thymus-dependent antigens and MHC gene products, either independently or as associated structures. We attempt to relate this complex specificity pattern to the regulatory and surveillance functions of T lymphocytes.

Allogeneic carrier-specific enhancement of hapten-specific secondary B-cell responses

We have analyzed the capacity of carrier-specific T cells to enhance the immune response of hapten-specific secondary B cells which do not share genes in the H-2 complex with the T cells. For this analysis we have used the in vitro splenic focus technique which allows assessment of monoclonal responses of B cells isolated in splenic fragment cultures of irradiated reconstituted carrier primed mice. A previous report from this laboratory demonstrated that syngeny in the I region of the H-2 complex was necessary between collaborating hapten-specific primary (nonimmune) B cells and carrier-specific T cells for responses yielding IgG1 but not IgM antibody. These findings lead up to postulate that the expression of I-region gene products on the surface of primary B cells and I-region syngeny with collaborating carrier-specific T cells were essential elements in the triggering events leading to IgG1 synthesis by primary B cells. The results presented in the present report indicate that, unlike primary B cells, the majority of secondary B cells can be stimulated to produce IgG1 antibody in carrier-primed allogeneic recipients. Although the enhancement of secondary IgG1 responses is slightly greater with syngeneic T cells, the allogeneic collaborative interaction requires both carrier priming of recipient mice and stimulation with the homologous hapten-carrier complex and thus appears to be specific. These findings clearly discriminate secondary from primary B cells and indicate that the mechanism of stimulation of secondary B cells to yield IgG1-producing clones differs fundamentally from the stimulation of primary B cells in that the requisite for I-region syngeny is obviated.