IgG1 Allotypes Research Articles

Identification of IgG1 and IgG3 Allotypes by PCR and Sanger Sequencing.

The immunoglobulin heavy constant gamma (IGHG) gene cluster encoding immunoglobulin G (IgG) subclasses is highly polymorphic, resulting in amino acid variation along the antibody constant heavy chain referred to as allotypes. IGHG1 and IGHG3 are the two most polymorphic IgG subclasses in humans, with 4 classical IgG1 allotypes and 13 allotypes described for IgG3, though recent studies suggest greater allelic diversity, especially in underrepresented ethnic populations. Polymerase chain reaction (PCR) and Sanger sequencing of IGHG amplicons allow for the identification of the single nucleotide polymorphisms (SNPs) responsible for the observed amino acid substitutions. Here, we provide a detailed protocol for the amplification of IGHG1 and IGHG3 segments by PCR, sample preparation for Sanger sequencing, and analysis of sequencing data to identify SNPs associated with different IgG1 and IgG3 allotypes.

The Influence of Human IgG Subclass and Allotype on Complement Activation.

Complement activation via the classical pathway is initiated when oligomeric Igs on target surfaces are recognized by C1 of the complement cascade. The strength of this interaction and activation of the complement system are influenced by structural variation of the Ab, including Ab isotype, subclass, and glycosylation profile. Polymorphic variants of IgG have also been described to influence Fc-dependent effector functions. Therefore, we assessed complement binding, deposition, and complement-dependent cytotoxicity (CDC) of 27 known IgG allotypes with anti-trinitrophenyl specificity. Differences between allotypes within subclasses were minor for IgG1, IgG3, and IgG4 allotypes, and more substantial for IgG2. Allelic variant IGHG2*06, containing a unique serine at position 378 in the CH3 domain, showed less efficient complement activation and CDC compared with other IgG2 polymorphisms. We also observed variable cell lysis between IgG1 and IgG3, with IgG3 being superior in lysis of human RBCs and Ramos cells, and IgG1 being superior in lysis of Raji and Wien133 cells, demonstrating that a long-standing conundrum in the literature depends on cellular context. Furthermore, we compared IgG1 and IgG3 under different circumstances, showing that Ag density and Ab hinge length, but not complement regulators, define the context dependency of Ab-mediated CDC activity. Our results point toward a variation in the capacity of IgG subclasses to activate complement due to single amino acid changes and hinge length differences of allotypes to activate complement, which might give new insights on susceptibility to infectious, alloimmune, or autoimmune diseases and aid the design of Ab-based therapeutics.

A Quantitative Approach to Unravel the Role of Host Genetics in IgG-FcγR Complex Formation After Vaccination.

Fc-mediated immune functions have been correlated with protection in the RV144 HIV vaccine trial and are important for immunity to a range of pathogens. IgG antibodies (Abs) that form complexes with Fc receptors (FcRs) on innate immune cells can activate Fc-mediated immune functions. Genetic variation in both IgGs and FcRs have the capacity to alter IgG-FcR complex formation via changes in binding affinity and concentration. A growing challenge lies in unraveling the importance of multiple variations, especially in the context of vaccine trials that are conducted in homogenous genetic populations. Here we use an ordinary differential equation model to quantitatively assess how IgG1 allotypes and FcγR polymorphisms influence IgG-FcγRIIIa complex formation in vaccine-relevant settings. Using data from the RV144 HIV vaccine trial, we map the landscape of IgG-FcγRIIIa complex formation predicted post-vaccination for three different IgG1 allotypes and two different FcγRIIIa polymorphisms. Overall, the model illustrates how specific vaccine interventions could be applied to maximize IgG-FcγRIIIa complex formation in different genetic backgrounds. Individuals with the G1m1,17 and G1m1,3 allotypes were predicted to be more responsive to vaccine adjuvant strategies that increase antibody FcγRIIIa affinity (e.g. glycosylation modifications), compared to the G1m-1,3 allotype which was predicted to be more responsive to vaccine boosting regimens that increase IgG1 antibody titers (concentration). Finally, simulations in mixed-allotype populations suggest that the benefit of boosting IgG1 concentration versus IgG1 affinity may be dependent upon the presence of the G1m-1,3 allotype. Overall this work provides a quantitative tool for rationally improving Fc-mediated functions after vaccination that may be important for assessing vaccine trial results in the context of under-represented genetic populations.

Novel approach to monitor intravenous immunoglobulin pharmacokinetics in humans using polymorphic determinants in IgG1 constant domains.

Clinical efficacy of intravenous immunoglobulin treatment (IVIg) is related to its pharmacokinetic (PK) profile. Its usual evaluation, by measuring serum total IgG levels, is imprecise, because IVIg cannot be distinguished from endogenous IgG. We developed ELISAs to specifically monitor the PK of IVIg using the polymorphic determinants G1m(a), G1m(x), and G1m(f). The specificity of the IgG1 allotype assays was sufficient to determine IVIg concentrations as low as 0.1 mg/mL in sera from individuals not expressing the respective markers. IVIg was quantified in posttreatment serum from patients with Guillain-Barré syndrome (GBS) by measuring IgG1 allotypes not expressed endogenously. After serotyping, 27/28 GBS patients were found eligible for IVIg monitoring using one or two genetic markers. In 17 cases, IVIg levels could be determined by both anti-G1m(a) and anti-G1m(x) measurement, showing significant correlation. Longitudinal monitoring of IVIg PK in seven GBS patients showed potential differences in clearance of total IgG versus IVIg-derived IgG, highlighting that total IgG measurements may not accurately reflect IVIg PK. To summarize, anti-IgG1 allotype assays can discriminate between endogenous IgG and therapeutic polyclonal IgG. These assays will be an important tool to better understand the variability in IVIg PK and treatment response of all patients treated with IVIg.

FcγR Binding and ADCC Activity of Human IgG Allotypes.

Antibody dependent cellular cytotoxicity (ADCC) is an Fc-dependent effector function of IgG important for anti-viral immunity and anti-tumor therapies. NK-cell mediated ADCC is mainly triggered by IgG-subclasses IgG1 and IgG3 through the IgG-Fc-receptor (FcγR) IIIa. Polymorphisms in the immunoglobulin gamma heavy chain gene likely form a layer of variation in the strength of the ADCC-response, but this has never been studied in detail. We produced all 27 known IgG allotypes and assessed FcγRIIIa binding and ADCC activity. While all IgG1, IgG2, and IgG4 allotypes behaved similarly within subclass, large allotype-specific variation was found for IgG3. ADCC capacity was affected by residues 291, 292, and 296 in the CH2 domain through altered affinity or avidity for FcγRIIIa. Furthermore, allotypic variation in hinge length affected ADCC, likely through altered proximity at the immunological synapse. Thus, these functional differences between IgG allotypes have important implications for therapeutic applications and susceptibility to infectious-, allo- or auto-immune diseases.

Immune responses to an inactivated influenza vaccine in Indigenous Australians

Abstract Indigenous Australians are highly susceptible to severe influenza disease during seasonal and pandemic infections. Current antibody-based vaccines targeting humoral immunity are the most effective way to combat influenza infections but show reduced efficacy in Indigenous Australians. The underlying mechanisms are still unknown. We recruited 166 Indigenous & non-Indigenous donors to elucidate adaptive immune responses induced by the inactivated quadrivalent influenza vaccine. Participants were bled at baseline, day 7 and day 28 post-vaccination. We analysed hemagglutination inhibition titres against the vaccine and previously circulating strains, analysed IgG1 and IgG3 allotypes, evaluated total IgG glycosylation and performed Luminex assays to dissect antibody responses against viral HA, NA and NP proteins. Additionally, we assessed influenza-specific B cell responses with fluorescent HA probes, antibody-secreting cells and circulating Tfh cells. More than 15,000 datapoints were collected to compare adaptive immune responses between Indigenous and non-Indigenous donors. We found that the majority of immune readouts were comparable between the two groups. However, interestingly, we found significantly lower pre-vaccine titres and lower back-boosting to pre-pandemic H1N1 viruses in Indigenous compared to non-Indigenous donors. This might explain, at least in part, high susceptibility of Indigenous Australians to the 2009 pandemic. Our comprehensive dataset will be used to understand and improve optimal vaccine responses in Indigenous and non-Indigenous populations globally.

Immunoglobulin G1 Allotype Influences Antibody Subclass Distribution in Response to HIV gp140 Vaccination.

Antibody subclasses exhibit extensive polymorphisms (allotypes) that could potentially impact the quality of HIV-vaccine induced B cell responses. Allotypes of immunoglobulin (Ig) G1, the most abundant serum antibody, have been shown to display altered functional properties in regard to serum half-life, Fc-receptor binding and FcRn-mediated mucosal transcytosis. To investigate the potential link between allotypic IgG1-variants and vaccine-generated humoral responses in a cohort of 14 HIV vaccine recipients, we developed a novel protocol for rapid IgG1-allotyping. We combined PCR and ELISA assays in a dual approach to determine the IgG1 allotype identity (G1m3 and/or G1m1) of trial participants, using human plasma and RNA isolated from PBMC. The IgG1-allotype distribution of our participants mirrored previously reported results for caucasoid populations. We observed elevated levels of HIV gp140-specific IgG1 and decreased IgG2 levels associated with the G1m1-allele, in contrast to G1m3 carriers. These data suggest that vaccinees homozygous for G1m1 are predisposed to develop elevated Ag-specific IgG1:IgG2 ratios compared to G1m3-carriers. This elevated IgG1:IgG2 ratio was further associated with higher FcγR-dimer engagement, a surrogate for potential antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) function. Although preliminary, these results suggest that IgG1 allotype may have a significant impact on IgG subclass distribution in response to vaccination and associated Fc-mediated effector functions. These results have important implications for ongoing HIV vaccine efficacy studies predicated on engagement of FcγR-mediated cellular functions including ADCC and ADCP, and warrant further investigation. Our novel allotyping protocol provides new tools to determine the potential impact of IgG1 allotypes on vaccine efficacy.

IgG1 Allotypes Influence the Pharmacokinetics of Therapeutic Monoclonal Antibodies through FcRn Binding

Because IgG1 allotypes might have different half-lives, their influence on infliximab (G1m17,1 allotype) pharmacokinetics was investigated in a group of spondyloarthritis patients. Infliximab was found to have a shorter half-life in patients homozygous for the G1m17,1 allotypes than in those carrying the G1m3 with no G1m1 (G1m3,-1) allotype. Because the neonatal FcR (FcRn) is involved in the pharmacokinetics of mAbs, the interaction of different IgG1 allotypes with FcRn was examined using cellular assays and surface plasmon resonance. G1m17,1 mAbs, such as infliximab and rituximab, were shown to bind more efficiently to FcRn and to be transcytosed better than the G1m3,-1 mAb cetuximab, which explains why infliximab is a better competitor for endogenous IgG1 in G1m3,-1 allotype-bearing patients. A set of four allotype variants of adalimumab (G1m17,1; G1m17,-1; G1m3,1; and G1m3,-1) was also tested for its binding to FcRn, revealing that the G1m3,1 variant, not present in commercial mAbs, binds more efficiently to FcRn and is transcytosed better than the other three variants, all of which are found in therapeutic mAbs.

NK Cell and Ig Interplay in Defense against Herpes Simplex Virus Type 1: Epistatic Interaction of CD16A and IgG1 Allotypes of Variable Affinities Modulates Antibody-Dependent Cellular Cytotoxicity and Susceptibility to Clinical Reactivation.

HSV-1 latently infects most humans, causing a variable clinical picture that depends, in part, on host genetic factors. Both IgG and its cellular FcRs, CD16A and CD32A-C (encoded by FCGR3A and FCGR2A-C, respectively, on chromosome 1), display polymorphisms that could affect their defensive function. Of potential relevance are a FCGR3A dimorphism resulting in CD16A-valine/phenylalanine-158 allotypes with different IgG affinity, variations conditioning NK cell expression of CD32B or CD32C, and IgG1 H chain (IGHG1) and kappa-chain (IGKC) polymorphisms determining allotypes designated G1m and Km. In this study, we assessed the contribution of Ig genetic variations and their interaction with FcR polymorphism to HSV-1 susceptibility, as well as their impact on NK cell-mediated Ab-dependent cellular cytotoxicity (ADCC). Our results show an epistatic interaction between IGHG1 and FCGR3A such that the higher affinity CD16A-158V/V genotype associates with an asymptomatic course of HSV-1 infection only in homozygotes for G1m3. Furthermore, CD16A-158V and G1m3 allotypes enhanced ADCC against opsonized HSV-1-infected fibroblasts. Conversely, Km allotypes and CD32B or CD32C expression on NK cells did not significantly influence HSV-1 susceptibility or ADCC. NK cells degranulating against immune serum-opsonized HSV-1-infected fibroblasts had heterogeneous phenotypes. Yet, enhanced ADCC was observed among NK cells showing a differentiated, memory-like phenotype (NKG2C(bright)NKG2A(-)CD57(+)FcRγ(-)), which expand in response to human CMV. These results extend our knowledge on the importance of immunogenetic polymorphisms and NK cell-Ab interplay in the host response against HSV-1 and point to the relevance of interactions between immune responses elicited during chronic coinfection by multiple herpesviruses.

THU0009 Reduced Clinical Improvement in Rheumatoid Arthritis Patients Showing IgG1 Allotype -Infliximab Incompatibility

BackgroundIgG1 allotypes (G1m) can induce antibodies in incompatible individuals. Allotypes 1 and 17 are always in the same IgG1 molecule, allotype 2 only appears in IgG1 with allotypes G1m1,17 but...

The contrasting IgG-binding interactions of human and herpes simplex virus Fc receptors.

A virally encoded, high-affinity Fc receptor (FcR) is found on herpes simplex virus type 1 (HSV-1) particles and infected cells where its binding of non-immune IgG protects cells from host-mediated lysis. Whilst mutation or aglycosylation of the IgG CH2 domain reduced binding to human FcR, the interaction with HSV-1 FcR was not affected. However, the HSV-1 FcR, unlike human FcR, discriminates between human IgG1 allotypes, being sensitive to changes at positions 214 (CH1) and 356/358 (CH3), away from its proposed binding site at the CH2-CH3 interface. The biological consequences are not known but this is the first evidence of a major functional difference between IgG1 allotypes.

The herpes simplex virus type 1 Fc receptor discriminates between IgG1 allotypes.

Herpes simplex virus type 1 (HSV-1) expresses a complex of two virally encoded glycoproteins, gE and gl, which is capable of binding nonimmune human IgG. The gE-gl complex has thus become known as an Fc receptor (FcR), which reportedly binds human IgG subclasses in the order IgG4 > IgG1 > or = IgG2 and does not bind IgG3 from many individuals. There is, however, allelic variation in the genes encoding the human IgG1 heavy chain constant region and this gives rise to allotypes of IgG1. Using recombinant monoclonal IgG molecules of known isotype and mutants thereof we have unexpectedly discovered that the HSV-1 FcR discriminates between IgG1 allotypes. This is evidence of functional differences between IgG1 allotypes that may account for their distribution in populations. Furthermore, these findings suggest HSV-1 FcR binding sites on the IgG molecule some distance from the proposed binding site in the CH2-CH3 domain interface.

The herpes simplex virus type 1 Fc receptor discriminates between IgG1 allotypes

Herpes simplex virus type 1 (HSV-1) expresses a complex of two virally encoded glycoproteins, gE and gI, which is capable of binding nonimmune human IgG. The gE-gI complex has thus become known as an Fc receptor (FcR), which reportedly binds human IgG subclasses in the order IgG4 > IgG1 ≥ IgG2 and does not bind IgG3 from many individuals. There is, however, allelic variation in the genes encoding the human IgG1 heavy chain constant region and this gives rise to allotypes of IgG1. Using recombinant monoclonal IgG molecules of known isotype and mutants thereof we have unexpectedly discovered that the HSV-1 FcR discriminates between IgG1 allotypes. This is evidence of functional differences between IgG1 allotypes that may account for their distribution in populations. Furthermore, these findings suggest HSV-1 FcR binding sites on the IgG molecule some distance from the proposed binding site in the CH2-CH3 domain interface.

Activation of complement by human IgG1 and human IgG3 antibodies against the human leucocyte antigen CD52.

Activation of the complement cascade by immunoglobulin G (IgG) plays a major role in the host defense against pathogens. Using recombinant human antibodies specific for the leucocyte antigen CD52, different allotypes of human IgG1 subclass were compared for their ability to activate human complement. In addition the roles of the different length hinge regions of IgG1 and IgG3 were investigated. It was found that the naturally occurring allotypes G1m(a,z) and G1m(f), and one artificially created isoallotype, G1m(null), did not significantly differ in their overall ability to cause cell lysis. However, some differences in binding of individual components of the classical activation pathway were detected. More of the complement component C1s seemed to be associated with the allotype G1m(f), although this did not result in an overall improvement in lytic potency. In this system the wild-type IgG3 was found to be less effective in complement lysis than IgG1. By shortening the hinge region of IgG3 to resemble that of an IgG1 antibody, increased complement binding was observed compared with that of wild-type IgG3 and the IgG1 allotypes. The overall lytic potency of the antibody was also improved compared with wild type IgG3 and it was also slightly more effective than the IgG1 allotypes.

HLA-DR-dependent variation of intrathecal IgG1 (Gm) allotype synthesis in multiple sclerosis.

Genetic susceptibility to multiple sclerosis (MS) in Caucasians was previously shown to be correlated to the presence of given alleles at the HLA-DR and Gm loci. We now demonstrate that the humoral immune response in MS central nervous system (CNS) is modulated by both loci: the levels of IgG1 subclass and IgG1 allotypes in cerebrospinal fluid of MS patients depend on both their Gm genotype and their HLA-DR2 or HLA-DR7 phenotype. That HLA-DR molecules may either participate in a preferential recruitment of IgG1 allotype-producing B cells in MS CNS or act after such a selective homing is discussed. These results demonstrate that both HLA and Gm loci are synergistically involved in the modulation of the humoral immune response.

Induction of rheumatoid antibodies in the mouse. Regulated production of autoantibody in the secondary humoral response.

A/J mice were found to produce autoreactive IgM anti-IgG1 in response to secondary immunization with a number of protein antigens. No anti-IgG1 was produced after a single such immunization, indicating that antigen: IgG1 antibody complexes were responsible for inducing the autoreactive response. The size of the anti-IgG1 response was in some cases massive and of the same order of magnitude as the response to the foreign immunizing material. No significant anti-IgG2a, anti-IgG2b, or anti-IgG3 response was found in mice producing anti-IgG1. Virtually all of the anti-IgG1 material produced was of the IgM class and bound to the Fc region of autologous IgG1. A component of the anti-IgG1 was shown to be able to distinguish between the two mouse IgG1 allotypes. These results suggest that self-reactive anti-IgG is a common component of the secondary immune response of mice that may have powerful physiological and immunoregulatory effects.

Multiple DNA fragment polymorphisms associated with immunoglobulin mu chain switch-like regions in man.

DNA probes containing the switch region (S) associated with the human immunoglobulin heavy chain mu gene were used to investigate polymorphisms in the germ-line human DNA. Six polymorphisms, detected by a single restriction enzyme (Sst I) are described. Linkage studies in 29 families show that five of the six polymorphisms, although relatively unassociated in random individuals, segregate in complete linkage one to the other and to Gm allotypes (markers on the heavy chain of IgG), while the sixth segregates independently. Altogether, when one considers the DNA markers at the five closely linked loci and the IgG1 and IgG3 heavy chain allotypes, 33 different haplotypes have been described; of these, 28 are detected by the DNA polymorphism alone. Study of 158-187 random haplotypes showed strong linkage disequilibrium only between one DNA polymorphism (Sst A) and Gm. Of the polymorphic Sst I loci, one, Sst E [associated with 2.2- to 2.7-kilobase (kb) fragments], is included in the mu chain S region (S mu); another, Sst A (6.8-7.4 kb), must be very close to the gamma 1-gamma 3 chain gene cluster. Based on studies of an IgE human myeloma, a third polymorphism, Sst C (4.8-5.5 kb), should map 3' of the active epsilon chain gene. An Sst I restriction enzyme map of phage clones carrying the two alpha chain genes indicates that Sst A and Sst C loci probably overlap with the alpha 1 and alpha 2 S regions, respectively. Both deletion/duplications and point mutations were detected.

Allotype suppression in the chicken. II. Suppression in homozygous chickens with antiallotype antibody and allotype-disparate B cells.

Injection of heterozygous (M-1a/M-1b, G1g/G-1i) B 14-line chickens with antisera directed against either IgM-1a or IgM-1b induced suppression of the relevant IgM-1 and genetically linked IgG-1 allotypes, whereas a mixture of anti-M-1a and anti-M-1b antibodies failed to produce allotype suppression. Injection of anti-M-1 antiserum into M-1 homozygous chickens induced only a transient delay of a few days in the appearance and rise of serum IgM-1 levels. However, suppression of host allotypes was induced by injecting M-1, G-1 homozygous neonatal or embryonal recipients with anti-M-1 antisera together with B locus- histocompatible allotype-disparate spleen, bone marrow or bursal cells. The active cell type were donor B cells, which established chimerism in the injected host, whereas peripheral blood T lymphocytes from agammaglobulinemic donors were ineffective. Allotype suppression was attributed to a homeostatic control mechanism which is exerted by normal B cells (but not T cells) over B cell recruitment in anti-M-1 antibody-treated, immature hosts.

Concentration of IgG1 and IgG2a allotypes in serum of nude and normal allotype-congenic mice.

BALB/c (C) and C57BL/6 (B6) allotype--congenic strains of mice (C, C.B, B6, B6.C) were made congenitally anthymic by the introduction of the mutant nude gene (nu). The resulting nude mice (C.nu, C.B.nu, B6.nu and B6.C.nu) were found to produce highly variable quantities of IgG1 and IgG2a. Whether these mice showed high, normal, or low quantities of IgG1 or IgG2a relative to their normal counterparts depended on their age and genetic background. In all nude mice, IgG1 production appeared more sensitive to the loss of T cells than did IgG2a production. An extreme example of this was the deficiency of IgG1 but not IgG2a in adult B6.nu and B6.C.nu mice. Substantial quantities of both IgG1 and IgG2a were found in adult C.nu and C.B.nu mice.

Allotype analysis of histocompatible and semiallogeneic B cell chimeric chickens.

SUMMARY The quantitative relationship between donor and host allotype synthesis was studied in B cell chimeric chickens. Juvenile, cyclophosphamide-treated B14A strain, or (B14A x CHA)F1 hybrid recipients were given injections of spleen, bone marrow, or bursal cells from B14C donors and serum IgM-1 and IgG-1 allotypes were monitored for several weeks after cell transfer. Donor and host-derived serum allotype concentrations were inversely related, i.e., high donor allotype levels were linked directly to suppression of host allotypes. Donor-derived Ig synthesis in chimeras ceased between 10 and 20 weeks after cell transfer and host allotype levels recovered concurrently. Lymphoid cells from chickens which had rejected donor cells and recovered their host allotype, failed to manifest any suppressive effect on B cells from allotype-matched donors in adoptive transfer experiments. These results agree with our previous conclusion that the mechanism which determines the reciprocal relationship between host and donor allotype synthesis is attributable to a nonimmune surveillance mechanism operating between the respective B cell populations. (B14A x CHA)F1 hybrid recipients were more resistant than MHC-compatible B14A hosts toward the engraftment of B14C B cells. Although chimerism was established in chickens pretreated with cyclophosphamide, it did not occur in sublethally irradiated recipients even when administered in conjunction with antithymus globulin. However, 300 rad abrogated the barriers against T cells, resulting in lethal graft-versus-host (GVH) reactions which failed to occur in untreated recipients. B14C bone marrow cells produced more severe host Ig suppression in F1 hybrid than in B14A recipients. In contrast to the temporary nature of B cell chimerism in histocompatible B14C → B14A pairs, donor-derived allotype levels in Fl hybrid chimeras showed a gradual increase up to 20 weeks after transplantation. We conclude that a mild form of GVH reaction which suppressed the recruitment of host B stem cells prevented the graft rejection and amplified the production of donor-derived immunoglobulins. Hybrid resistance exhibited by Fl hybrid mice against parental haemopoietic cells or tumours is considered to be distinct from classical alloimmune reactions (1). Previous studies have been concerned with genetic control (2), the effector system represented probably by “natural killer” cells (3) and with agents that are capable of abrogating the host barrier (4–6). We have reported recently that juvenile chickens express resistance toward transplanted MHC-compatible B cells and that engraftment in cyclophosphamide (CY)-conditioned recipients resulted in suppression of host-derived Ig synthesis (7). These findings were attributed to a B cell surveillance mechanism similar to that which has been postulated previously to explain the host resistance toward haemopoietic and tumour cells. This paper presents a quantitative and kinetic analysis of donor and host-derived Ig synthesis in B locus (MHC)-compatible and parental → Fl hybrid cell chimeras.