RNA conversion of lymphoid cells to synthesize allogeneic immunoglobulins in vivo

Ribonucleic acid extracts (“5 day immune” and “nonimmune”-RNA) obtained from lymph nodes and spleens of rabbits homozygous for the b⁴ or b⁵ allele of light chain immunoglobulin allotypes were injected iv into nonimmunized rabbits homozygous for the alternate allele. The recipient rabbits were then given multiple iv injections of sheep red blood cells (SRBC). The spleens were assayed 13, 21, and 37 days following the RNA injection for “direct” IgM and “indirect” IgG plaque forming cells (PFC) specific for SRBC. The b4 or b5 light chain allotype and the a1, a2, and a3 heavy chain allotype of the antibody in the plaques was identified by radioautography and by inhibition of plaque formation using anti-allotype antibodies. The b light chain allotype of the RNA donor was identified in 22–32% of the IgM plaques and in 25–42% of the IgG plaques. The allotype of the host rabbit b light chain allotype was identified in 56–67% of the IgM plaques and in 57–71% of the IgG plaques. Likewise the a heavy chain allotype of the RNA donor was identified in 10–19% of the IgM plaques and in 12–19% of the IgG plaques. The allotype of the host rabbit a heavy chain allotype was identified in 51–60% of the IgM plaques and in 55–63% of the IgG plaques. The concentrated lysates of spleen and lymph node cells were also analyzed for immunoglobulins of each light chain allotype by immunodiffusion with radiolabeled antibody. The allotype of both the RNA donor rabbit and host rabbit were found in most of the lysates of lymphoid tissues and in some of the IgG isolated from the serum and concentrated.     

Partial amino acid sequence and genetic control of latent a2 allotype induced in rabbits by immunization with anti-a2 antibody

We have shown that after immunization of homozygous a1 rabbits of the B immunoglobulin (Ig) heavy chain haplotype with anti-a2 antibody (Ab) a population of molecules appears that has all of the serologic characteristics of the a2 allotype. We have now isolated these putative latent a2 molecules, have separated the heavy chains, and after enzymatic deblocking, have determined the first 19 N-terminal amino acids. For all eight allotype-associated residues, these putative latent a2 molecules have the amino acid residues typical of a2 allotype. As expected, the preimmune IgG from this a1a1 rabbit has the amino acids typical of the a1 allotype. Thus by partial amino acid sequence analysis, we provide additional evidence that the latent a2 allotype can be induced in a1a1 rabbits of the B heavy chain haplotype by immunization with anti-a2 Ab. Rabbits of other heavy chain haplotypes were also immunized with anti-a2 Ab and were tested for their ability to synthesize latent a2 allotype. Thus far, a1a1 rabbits of the A, B, C, and I heavy chain haplotypes all synthesize latent a2 allotype. In contrast, a3a3 rabbits of the G and H heavy chain haplotypes did not synthesize latent a2 allotype.     

The structural basis of rabbit VH allotypes: serologic studies on a1 H chains with defined amino acid sequence.

The amino acid sequences for the VH regions of three homogeneous antibodies elicited by type III pneumococcal vaccine were determined. All three antibodies had the group a allotype a1. Two of the antibody H chains (3372, 3381) had identical amino acid sequences in all framework positions that are considered correlates of the VH allotype, whereas the third H chain (3T72) differed from these at positions 15 and 16. The a1 allotypic specificities of the three homogeneous antibodies were compared by quantitative radiobinding and inhibition assays by using both insolubilized anti-a1 antisera and allotypic antiserum fractions rendered specific for the homogeneous antibody 3374. It was found that antibodies 3374 and 3381 are allotypically indistinguishable and have in common an a1 allotypic specificity that predominates in pooled a1 IgG. The allotypic specificity of the 3T72 antibody, on the other hand, was markedly deficient to those of 3374, 3381, and the a1 IgG pool. This correlation of allotypic difference with amino acid sequence variation at position 15 and 16 of the H chain indicates the involvement of these two residues in a major a1 allotypic determinant.     

Influence of the Ig H chain locus on autoantibody production in autoimmune mice.

Autoimmune disease is influenced by multiple genes. In this study, we investigated the role of one genetic locus, Ig H chain. IgG2a antichromatin, anti-ssDNA, and antihistone autoantibodies (autoAb) from (MRL/Mp-lpr/lpr x C57BL/6-lpr/lpr), (Ighj/b); (C57BL/6-lpr/lpr x C57BL/6-lpr/lpr-Igha), (Ighb/a); and (MRL/Mp-lpr/lpr x MRL/Mp-lpr/lpr-Ighb), (Ighj/b) mice were determined using allotype-specific ELISA. Strikingly, antichromatin and antihistone antibodies (Ab) were comprised of significantly more b allotype than either a or j allotype in all cohorts of F1 mice examined. In mice that produced anti-Sm Ab, the b allotype was used preferentially for these autoAb as well. However, no allotype skewing was observed in IgG2a Ab directed against TNP or DNA, or for total IgG2a. An Igh recombinant locus was utilized to examine the genetic control of b allotype skewing in lpr mice and in chronic graft vs host disease. In both models, the VH region did not appear to be responsible for the preferential use of b allotype. These results indicate a contribution to autoimmunity by the Igh locus and raise the possibility that Ig allotype may influence autoimmune disease by its effect on the production of certain autoAb.     

Human immunoglobulin allotypes

More than twenty recombinant monoclonal antibodies are approved as therapeutics. Almost all of these are based on the whole IgG isotype format, but vary in the origin of the variable regions between mouse (chimeric), humanized mouse and fully human sequences; all of those with whole IgG format employ human constant region sequences. Currently, the opposing merits of the four IgG subclasses are considered with respect to the in vivo biological activities considered to be appropriate to the disease indication being treated. Human heavy chain genes also exhibit extensive structural polymorphism(s) and, being closely linked, are inherited as a haplotype. Polymorphisms (allotypes) within the IgG isotype were originally discovered and described using serological reagents derived from humans; demonstrating that allotypic variants can be immunogenic and provoke antibody responses as a result of allo-immunisation. The serologically defined allotypes differ widely within and between population groups; therefore, a mAb of a given allotype will, inevitably, be delivered to a cohort of patients homozygous for the alternative allotype. This publication reviews the serologically defined human IgG allotypes and considers the potential for allotype differences to contribute to or potentiate immunogenicity.     

Human immunoglobulin allotypes: Possible implications for immunogenicity

More than twenty recombinant monoclonal antibodies are approved as therapeutics. Almost all of these are based on the whole IgG isotype format, but vary in the origin of the variable regions between mouse (chimeric), humanized mouse and fully human sequences; all of those with whole IgG format employ human constant region sequences. Currently, the opposing merits of the four IgG subclasses are considered with respect to the in vivo biological activities considered to be appropriate to the disease indication being treated. Human heavy chain genes also exhibit extensive structural polymorphism(s) and, being closely linked, are inherited as a haplotype. Polymorphisms (allotypes) within the IgG isotype were originally discovered and described using serological reagents derived from humans; demonstrating that allotypic variants can be immunogenic and provoke antibody responses as a result of allo-immunization. The serologically defined allotypes differ widely within and between population groups; therefore, a mAb of a given allotype will, inevitably, be delivered to a cohort of patients homozygous for the alternative allotype. This publication reviews the serologically defined human IgG allotypes and considers the potential for allotype differences to contribute to or potentiate immunogenicity.Key words: human IgG, polymorphisms, IgG allotypes, antibody therapeutics, immunogenicity, anti-therapeutic antibody, IgG glycosylation     

Comparison of the amino acid sequences of the variable domains of two homogeneous rabbit antibodies to type III pneumococcal polysaccharide.

The amino acid sequences of the V (variable) regions of the H (heavy) and L (light) chains derived from rabbit antibody K-25, specific for type III pneumococci, were determined; this is the second homogeneous rabbit antibody besides antibody BS-5 whose complete sequence of the V domain has been established (Jaton, 1974d). The V regions of L chains BS-5 and K-25 (both of allotype b4) differ from each other by 19 amino acid residues; 11 of these 19 substitutions are located within the three hypervariable sections of the V region. On the basis of seven amino acid differences within the N-terminal 28 positions, it is suggested that L chain K-25 belongs to a different subgroup of rabbit K chains and L chain BS-5. H chain K-25 (allotype a2) differs from another H chain of the same allotype by one amino acid substitution within the N-terminal 70 positions in addition to interchanges occurring in the first two hypervariable sections. H chain K-25 was compared with H chain BS-5 (allotype a1) and with the known V-region rabbit sequences. Allotype-related differences between a1, a2 and a3 chains appear to occur within the N-terminal 16 positions and possibly in scattered positions throughout the V-region. In the hypervariable positions, variability between the two antibodies is remarkably more pronounced within the third hypervariable section of both H and L chains than within the first two.     

Optimal strategies in immunology III. The IgM-IgG switch

Summary During a primary immune response generally two classes of antibody are produced, immunoglobulin M (IgM) and immunoglobulin G (IgG). It is currently thought that some lymphocytes which initially produce IgM switch to the production of IgG with the same specificity for antigen. During a secondary immune response IgG is the predominant antibody made throughout the response. In this paper we address the question of why such apparently complicated modes of response should have been adapted by evolution.We construct mathematical models of the immune response to growing antigens which incorporate complement dependent cell lysis. By comparing the times required to eliminate antigen we show that under certain conditions it is advantageous for an animal to switch some of its lymphocytes from IgM to IgG production during a primary response, but yet to secrete only IgG during a secondary response. The sensitivity of such a conclusion to parameter variations is studied and the biological basis and implications of our models are fully discussed.Portions of this work were performed under the auspices of the U.S. Department of Energy. A.S.P. was also supported by the National Science Foundation under Grant No. ENG-7904852 and BRSG grant S07 RR05664-11 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institute of Health. A.S.P. is the recepient of an NIH Research Career Development Award 1K04 AI 00357-01. S.R. was a recipient of NIH Fellowship 5 F32 AI05107-02     

Serologic and structural characterization of immunoglobulin chains secreted by rabbit-mouse hybridomas

Serologic and primary structural analyses of Ig chains secreted by several rabbit-mouse hybridomas have shown that these hybrid cells produce heavy (H) or light (L) chains identical to those isolated from rabbit sera. Two of the cell lines (7D2, 7D6) secreted rabbit H chains with a m.w. of 55,000 each of which expressed a full complement of variable and constant region allotypes (a3, d11, e15). These apparently normal rabbit H chains were secreted in a complex with a m.w. about 130,000, and serologic studies indicated that this complex contained a covalently linked mouse kappa L chain. Two other cell lines (4C1, 12F2) produced allotype b4 L chains with m.w. of 23,000 and 25,000, and a third (1D4P5) produced an allotype b5 L chain with a m.w. of 23,000. Serologic analyses indicated that the allotypes on these chains are equivalent to those expressed by normal rabbit Ig molecules. Partial amino acid sequence data obtained for the L chain products showed them to be typical of rabbit L chains, and to be significantly different from mouse L chains.     

Rapid stimulation of large specific antibody responses with conjugates of antigen and anti-IgD antibody

Injection of mice with goat anti-mouse IgD antibody stimulates a large IgG1 anti-goat IgG antibody response, as well as polyclonal IgG1 production. To determine if this phenomenon could be used to induce large antibody responses to other Ag, covalent conjugates were produced between BSA or other Ag and H delta a/1, a mAb specific for IgD of the a allotype, and between BSA and AF3.33, a mAb specific for IgD of the b allotype. Injection of H delta a/1-BSA into BALB/c mice, which express Ig of the a allotype, or into (BALB/c x CB20)F1 mice (a x b allotype heterozygotes) induced IgG1 anti-BSA antibody responses that peaked 8 to 9 days after injection, and were more than 1000 times larger than those induced by injection of BSA alone, and 100 times larger than those induced by injecting unconjugated BSA plus H delta a/1. H delta a/1-BSA was no more immunogenic than unconjugated BSA when injected into CB20 mice, which express Ig of the b allotype, while AF3.33-BSA greatly enhanced anti-BSA antibody production in CB20, but not in BALB/c mice. Mice serially immunized with three different Ag conjugated to H delta a/1 made large antibody responses to all three Ag, provided that the mouse strain used did not recognize allotypic determinants on H delta a/1 as foreign and produce a neutralizing antibody response. Intravenous and s.c. routes of inoculation produced responses of similar magnitude and relatively low variability; responses to footpad or intramuscular inoculation were more variable, and i.p. inoculation induced smaller responses. Injection of BALB/c mice i.v. with 100 micrograms of H delta a/1-BSA induced an IgG1 anti-BSA response of 5.6 mg/ml, which was approximately 70% of the total IgG1 response. Anti-BSA responses to 30 micrograms of conjugate or less were much smaller, but could be considerably enhanced by adding unconjugated H delta a/1 to the inoculum. This system will be useful for the rapid stimulation of large antibody responses to biologically important Ag, and for investigating mechanisms of Ag processing and B and T cell activation.     

Alteration of the B cell surface phenotype, immune response to phosphocholine and the B cell repertoire in M167 mu plus kappa transgenic mice

M167, mu plus kappa, transgenic mice have been analyzed for the expression of the transgene product as a cell surface, Ag-specific receptor and for their ability to respond to Ag. The vast majority of B cells in these H + L transgenics (97 to 99%) express large amounts of the transgene product on their surface and are capable of binding phosphocholine. A total of 4 to 30% of the B cells also express endogenous IgM and IgD H chain products. After immunization with phosphocholine (PC)-conjugated keyhole limpet hemocyanin, more than 1000 micrograms/ml of anti-PC antibody bearing the transgene IgMa allotype marker are produced. Surprisingly, significant amounts of anti-PC antibodies that express the endogenous, IgMb allotype, are also produced; however, these antibodies lack the T15-idiotype which dominates the anti-PC response in their nontransgenic littermate controls. The B cells producing these endogenous anti-PC antibodies also fail to switch to IgG anti-PC synthesis, whereas B cells producing anti-keyhole limpet hemocyanin antibodies readily undergo class switching. These last two observations may be due to the fact that the endogenous anti-PC antibody actually results from mixed mu a + mu b molecules in which the transgene encoded H and L chains are most likely responsible for the binding of PC. Thus, a switch of the endogenous isotype from mu b to IgG would result in a loss of specificity for PC in the IgG molecules produced using the endogenous VH-gene product(s), and mu a + gamma b hybrid molecules are not likely to be formed. This hypothesis is supported by the fact that the majority of (mu a + mu b) hybridomas have the mu b-allotype joined with a VH region other than the VH1 gene which is required for PC-binding and T15 idiotype expression.     

Immunogenetic study of porcine IgG

Three allotypes of IgG were identified in pig. Based on data obtained on electrophoretic mobility as well as on results of the International Comparative Test ISABR for pig blood group, polymorphic proteins and enzymes (1987-1988), the allotypes are specified as markers of two different IgG subclasses and are referred to as IgG1a, IgG2b and IgG2c. The former of these is established as corresponding to the already known IGH3 C1, and the other two had not been earlier described. In herds of pigs being bred in the Georgian SSR, the IgG1a allotype frequencies in Kakhetinskaya, Large White, Landrase and Lithuanian white were 0.84, 0.93, 0.91 and 0.94, respectively, whereas for the IgG2b allotype it ran 0.89, 0.73, 0.79, and 0.69 in the order mentioned. The IgG2c allotype was not registered in samples under examination.     

Immunoelectron microscopic localization of idiotypes and allotypes on immunoglobulin molecules

We have developed a novel approach to the analysis of antigenic (allotypic and idiotypic) determinants on intact immunoglobulin molecules. Immune complexes composed of IgG in combination with anti-idiotype or anti-allotype antibody were "visualized" by transmission electron microscopy. Individual Fab fragments of anti-idiotype or anti-allotype antibody, when bound to the IgG, altered the "Y" configuration in a reproducible and interpretable manner. Anti-idiotype antibody (either as Fab or IgG) bound to the terminus of the presumed V region of the IgG molecule, thus extending the apparent length of the Fab arms. Analysis of a rabbit VH framework allotype (a1) revealed that the determinant(s) is (are) located on the lateral portion of the V region of IgG. Binding of the anti-a1 Fab fragments was always at approximately right angles to the axis of the Fab arms of IgG. Fab antibody to the rabbit kappa light chain (b4) allotype bound to the lateral portion of the terminal half of the IgG Fab arms. This technique should be of value in localizing less well defined immunoglobulin determinants.     

The modification of human immunoglobulin binding to staphylococcal protein A using diethylpyrocarbonate

Human IgG subclasses 1, 2, and 4, as well as proteins of the IgG3 subclass that are allotype G3m (s+t+), bind avidly to staphylococcal protein A by means of their Fc portion. Proteins of the IgG3 subclass that are allotype G3m (s-t-) do not bind. The importance of a histidine residue at position 435 has been implicated from comparison of amino acid sequences of immunoglobulins that bind with those that do not bind to staphylococcal protein A, as well as from crystallographic data. Modification of histidines at a low concentration of diethylpyrocarbonate successfully and reversibly alters the binding of immunoglobulins to staphylococcal protein A with only minimal change in the antigenic properties. This method provides strong evidence for the critical importance of histidine in the binding of immunoglobulins to staphylococcal protein A.     

T cell clones specific for IgG2a of the a allotype: direct evidence for presentation of endogenous antigen

T cell clones specific for IgG2a of the alpha allotype have been isolated from C57BL/6J mice. Antigenic determinants recognized by these clones were localized by using a panel of hybrid IgG2b-IgG2a myeloma proteins. These experiments provide evidence for two distinct antigenic sites, one located in a segment encompassing the hinge region and most of the CH2 domain, and the other in a segment spanning the CH3 domain and the C-terminal eight residues of the CH2 domain. As judged by their failure to respond in the presence of B6.C-H-2bm12 spleen cells, all the clones recognize determinants created, in part, by the I-A beta chain. A strong proliferative response was observed in the presence of spleen cells from several H-2b strains, including C3H.SW, A.BY, D1.LP, and BALB.B. Experiments testing reactivity directed toward spleen cells from appropriate allotype-congenic mouse strains demonstrated that this response was controlled by Igh-linked genes. These results clearly indicated 1) that Igh-1a-specific T cell clones are stimulated by endogenously synthesized IgG2a, and 2) these T cells recognize shared determinants expressed on IgG2a molecules of various strains. These experiments thus provide strong evidence for presentation of self antigens under normal physiological conditions.     

Inheritance of idiotype expression unlinked to the H chain allotype locus. Novel features of genetic control in the Ia.7 system

We have observed a pattern of inherited idiotype expression in three mouse strains that is unexpected from the genetics of the strains: a dominant idiotype that was expressed at high levels in two parental strains was expressed only at low levels in a heavy chain allotype congenic strain derived from them. In the C3H.SW strain, the antibody response to the class II MHC Ag I-E is of limited diversity, with dominant expression of an idiotype and the V kappa 21 L chain. The C57BL/10 strain expresses the same idiotype at high levels, whereas the CWB/12 strain, which was derived by replacing the Ig H chain Igh-Cj allele of C3H.SW with the Igh-Cb allele derived from C57B1/10, has been found to express little of this dominant idiotype. CWB/12 responds, with titers equal to those of the parental strains, to the I-E epitope responsible for dominant idiotype expression, and it expresses normal V kappa 21 levels; thus deficiencies in epitope-specific responsiveness or in V kappa 21 expression cannot explain the low Id expression in CWB/12. Furthermore, Southern blot analysis of three VH families gave no evidence of recombination within the the VH locus of CWB/12, which was Igh-Vb throughout. Black-cross analysis demonstrated that expression of the dominant idiotype segregated independently of Ig allotype, and was therefore due to genes unlinked to the H chain gene locus. To our knowledge, this pattern of Id expression is unprecedented, and indicates the need for caution in the interpretation of studies using allotype congenic strains. It also demonstrates a role for genes outside the Igh locus in the control of Id expression.     

Complement C1q binding affects spin-labeled heterosaccharides of rabbit antibodies in immune but not artificial immunoglobulin G aggregates

IgG anti-hapten antibodies were purified from the sera of rabbits homozygous for allotypic determinants d11 and d12 in the constant region of the heavy chain. Correlative with this determinant is the absence (d11) or presence (d12) of an oligosaccharide chain just below the hinge region of the IgG molecule. Both d11 and d12 molecules contain a complex heterosaccharide chain located near the carboxyl terminus of the second constant region domain. The two populations of IgG antibodies were thus selectively labeled with the spin probe Tempamine in their second constant region domains by reductive amination primarily of terminal N-acetylneuraminic acid residues. Chemical and enzymatic cleavages showed about 80% of the attached spin labels were N-acetylneuraminic acid-associated. Analysis of probe adducts by ESR spectrometry showed the presence of slower and faster moving subcomponents. Formation of immune complexes by antigen induces slight but significant restrictions of spin label mobility for both d11 and d12 IgG molecules. This restriction is qualitatively different from that seen in glutaraldehyde-, carbodiimide-, or ethanol-induced aggregates of the same IgG antibodies. The addition of purified complement C1 subcomponent C1q to immune aggregates resulted in marked immobilization of spin labels, the rotational correlation time of which was 30-40 mu s for both d11 and d12 molecules (evaluated by saturation transfer spectroscopy). A similar spin probe immobilizing effect is not seen when C1q binds to chemically aggregated IgG antibodies (which also do not activate C1). A novel model is proposed in which C1q is hypothesized to juxtapose Fc moieties in a discrete fashion required for subsequent C1 activation processes mediated by immune complexes.     

A murine–human chimeric IgG antibody against vascular endothelial growth factor receptor 2 inhibits angiogenesis in vitro

Vascular endothelial growth factor receptor 2 (VEGFR2) has been reported to play an important role in angiogenesis and tumorigenesis. A murine anti-VEGFR2 mAb (A8H1) has been established in a previous study. To reduce the incompatibility of the murine mAb for human use, the chimeric anti-VEGFR2-IgG was developed by genetic recombination of the variable regions of the A8H1 antibody and the constant regions of human IgG, and was expressed in Sp2/0 cells transfected with the two recombinant vectors containing the heavy chain and the light chain regions. After screening, clone 2F12 was selected and was found to stably secrete the murine–human chimeric anti-VEGFR2-IgG (coded 2F12). This chimeric IgG maintained the specificity and the affinity of the parental murine antibody against VEGFR2, and effectively identified VEGFR2 expressed on the surface of HUVECs and BEL-7402 cells. Furthermore, the 2F12 antibody demonstrated inhibition of angiogenesis in vitro, such as proliferation, migration, invasion and tube formation of HUVECs. This murine–human chimeric IgG may be considered for further development as an anti-angiogenesis and anti-tumor agent.     

Allotypically related sequences in the Fd fragment of rabbit immunoglobulin heavy chains

The sequence has been completed of the N-terminal 94 residues of the variable section of the Fd fragment of heavy chains from rabbit immunoglobulin G (IgG) of allotype As1. Most of the sequence of the same section from IgG of allotype Aa3 is also reported. These results, in conjunction with a substantial sequence of the variable region of allotype Aa2 reported elsewhere (Fleischman, 1971), show the presence of 16 positions (including six consecutive positions) in which the residue present correlates with the allotype. No allotype-related sequence variation has been found in the constant section of the Fd fragment. This evidence supports the view that two genes code for the heavy chain and it can be used as evidence in favour of somatic mutation as the origin of the variability in the sequence of the N-terminal section. The evolutionary origin of the ;a' locus allotypes of rabbit immunoglobulins remains obscure.     

Role of antigen-specific T cell help in the generation of in vivo antibody responses. I. Antigen-specific T cell help is required to generate a polyclonal IgG1 response in anti-IgD antibody-injected mice.

A system in which injection of mice with an antibody to mouse IgD that they recognize as foreign stimulates a large, T cell-dependent IgG response was used to study whether Ag-specific T cell help is required to stimulate polyclonal (non-Ag-specific) IgG production in vivo. Igha x Ighb allotype heterozygous mice were injected with a conjugate of a foreign Ag coupled to a mAb specific for one of the two IgD allotypes expressed in these mice. This conjugate cross-links mIgD on B cells that express the recognized allotype. These cells process the conjugate and present the foreign Ag to Ag-specific T lymphocytes, which become activated. Thus, B cells of the recognized allotype can be stimulated by cross-linking of their mIgD, Ag-specific T cell help, non-Ag-specific cytokines, and non-Ag-specific contact with activated T cells. In contrast, B cells that express the Igh allotype not recognized by the Ag-anti-IgD antibody conjugate (bystander B cells) can be stimulated in this system only by non-Ag-specific cytokines and non-Ag-specific contact with activated T cells. Although both recognized and bystander B cells in conjugate-injected mice demonstrated substantial increases in size and Ia expression, only the recognized B cells were induced to synthesize DNA and to make a substantial polyclonal Ig response. Bystander B cells still failed to secrete IgG when mice were injected with an anti-IgD-Ag conjugate specific for the other Igh allotype as well as a mAb that cross-linked IgD of the bystander B cell allotype. These observations demonstrate that although non-Ag-specific cytokine and contact-mediated T cell help are sufficient to induce B cells to increase in size and Ia expression in anti-IgD antibody-injected mice, Ag-specific T cell help is required to stimulate the generation of an IgG response in these mice.