Determinants recognized by murine rheumatoid factors: molecular localization using a panel of mouse myeloma variant immunoglobulins

The Absence of CD44 Ameliorates Faslpr/lprDisease

The lpr mutation in the Fas gene leads to symptoms of autoimmune disease, including polyclonal B-lymphocyte activation and lymphoproliferation. The expanding T-lymphocyte populations in the disease are characterized by high expression levels of the memory marker CD44. It has not been known whether CD44 expression contributes to the pathophysiology of the condition or merely reflects a consequence of excessive lymphocyte activation. We therefore tested the role of CD44 gene products in Fas(lpr/lpr) disease in gene targeted mice. We bred CD44 knockout mice and C57Bl/6-lpr mice to generate the Fas(lpr/lpr) CD44+/+, and Fas(lpr/lpr) CD44-/- genotypes and analyzed the disease manifestations around 215 days of age. The absence of CD44 substantially reduced the immunoglobulin secretion and lymphocyte expansion that are characteristic of the Fas(lpr/lpr) syndrome. Surprisingly, the percentage of CD3+ CD4- CD8- (double negative) cells in peripheral lymphoid organs increased in Fas(lpr/lpr) CD44-/- mice almost to the same extent as in Fas(lpr/lpr) CD44+/+ mice. These results indicate that the expansion of the fraction of double negative cells in spleens and lymph nodes, believed to be generated by down-regulation of CD8, does not depend on increased lymphocyte numbers. Furthermore, they corroborate an essential role for CD44 gene products in the T-cell expansion and polyclonal B-cell activation that constitute the Fas(lpr/lpr) syndrome. The CD44 receptor may be a suitable therapeutic target for inhibition of lymphoproliferation in autoimmune diseases.

Differential roles of osteopontin/Eta-1 in early and late lpr disease

The cytokine osteopontin (Eta-1) leads to macrophage-dependent polyclonal B-cell activation and is induced early in autoimmune prone mice with the lpr mutation, suggesting a significant pathogenic role for this molecule. Indeed, C57BL/6-Fas(lpr/lpr) mice crossed with osteopontin(-/-) mice display delayed onset of polyclonal B-cell activation, as judged by serum immunoglobulin levels. In contrast, they are subject to normal onset, but late exacerbation of lymphoproliferation and evidence of kidney disease. These observations define two stages of Fas(lpr/lpr) disease with respect to osteopontin-dependent pathogenesis that should be taken into account in the design of therapeutic approaches to the clinical disease.

Activation of effector functions by immune complexes of mouse IgG2a with isotype-specific autoantibodies

Analysis of five monoclonal autoantibodies, rheumatoid factors produced by hybridomas generated from spleen cells of BALB/c mice repeatedly infected with A/PR/8/34 human influenza A virus, revealed that they recognized distinct but spatially related epitopes. The differing isoallotypic specificity of the IgM and IgA monoclonal antibodies correlated with the presence of Ile258 and Ala305, respectively. Although these data suggest that the epitopes recognized are within the CH2 domain, all antibodies failed to inhibit IgG antigen reactivity with Staphylococcus aureus protein A (SpA), C1q, mouse C3, human Fc gamma RI or mouse Fc gamma RII, activities known to be predominantly determined by CH2 domain structures. Reactivity of the IgA antibody, Z34, with IgG2b allowed further specificity studies using a panel of 26 mutant IgG2b proteins, each having single amino acid replacements over the surface of the CH2 domain. The only substitution that affected Z34 reactivity was Asn/Ala297, which destroyed the glycosylation sequon, resulting in secretion of an aglycosylated IgG molecule. The epitope recognized by Z34 therefore seems to be located outside of the Fc gamma R and C1q binding sites, but to be dependent on the presence of carbohydrate for expression. In contrast to the binding studies, complement activation by aggregated IgG2a, through classical or alternative pathways, was inhibited by the presence of autoantibodies. The functional significance of isotype-specific autoantibody in immune regulation is discussed.

Regulation of antibody production mediated by Fc gamma receptors, IgG binding factors, and IgG Fc-binding autoantibodies

Fc receptors (FcRs) are immunoglobulin-binding structures that enable antibodies to perform a variety of functions by forming connections between specific recognition and effector cells. Besides eliciting cytotoxicity, inducing secretion of mediators and endocytosis of opsonized particles, FcRs are involved in the regulation of antibody production, both as integral membrane proteins and as soluble molecules released from the cell surface. Most FcRs belong to the same family of proteins as their ligands (immunoglobulin superfamily). This review contains recent data obtained by use of monoclonal antibodies and cloning studies on FcRs and FcR-like molecules. The importance of fine specificity of receptor binding site(s)--that of the conformation of FcRs and their ligands in triggering signaling mechanisms--is analyzed. The regulatory function of membrane-bound and -released FcRs; the correlation between cell cycle, FcR expression, and release; as well as the possible mechanisms of these phenomena are discussed.

Intravascular metabolism of normal and mutant mouse immunoglobulin molecules

The metabolism of IgG immunoglobulins in the body is tightly regulated in order to maintain their intravascular concentration. Different subclasses may have different intravascular half-lives, and in the mouse, passively administered IgG2b disappears from the circulation more rapidly than IgG2a. We have attempted to localize the sequences in the constant region which are responsible for this difference by examining the intravascular metabolism of mutant immunoglobulins that were generated in tissue culture and have undergone deletions of individual constant region domains or contain different combinations of gamma 2b and gamma 2a CH2 and CH3 domains. Our results suggest that the regulation of intravascular metabolism is complex but indicate that sequences in the CH3 domain are important in determining the different intravascular half-lives of IgG2b and IgG2a antibodies in the mouse.

Heterogeneity in rheumatoid factor isotypes and specificities in MRL mice

MRL/lpr mice spontaneously develop an arthritis which, in several respects, is similar to human rheumatoid arthritis, including joint inflammation and circulating rheumatoid factors. In human disease, circulating IgM rheumatoid factor (RF) predominates but, surprisingly, in these mice we have detected much more IgA rheumatoid factor. This IgA rheumatoid factor has a major specificity for IgG2a, but heterogeneity in binding specificity was seen between different mice. IgG rheumatoid factors were determined in a heterologous mouse IgG assay, in which each subclass of rheumatoid factor was tested for its ability to bind to the remaining IgG subclasses. Rheumatoid factor activity was detected in all the IgG subclasses, but particularly elevated levels were seen in IgG3 and IgG1. Both the levels and specificities of IgG rheumatoid factors were markedly different between each mouse.

Variable region sequences of murine IgM anti-IgG monoclonal autoantibodies (rheumatoid factors). II. Comparison of hybridomas derived by lipopolysaccharide stimulation and secondary protein immunization

We have obtained the complete variable region mRNA sequences of 11 LPS-derived and 14 secondary immunization-derived monoclonal IgM anti-IgG antibodies (rheumatoid factors, RFs). A comparative analysis of these sequences showed that monoclonal RFs derived after polyclonal activation are structurally very similar to RFs derived after secondary protein immunization. This study was undertaken to evaluate the potential relationship between two previously described phenomena: (a) during a secondary response to a protein antigen, RF is produced in quantities that equal or exceed the immunogen-specific antibody; and (b) the frequency of B cells that make RF after polyclonal activation is quite high; 3-10%. It has been unclear whether LPS-stimulated cells that produce IgM anti-IgG that is detected by an in vitro assay are related to the cells that produce RF after in vivo stimulation. The similarity of the antigen receptors found in the two types of RF, however, suggests that most or all of the RF-producing B cells detected after LPS stimulation would also be stimulated during the secondary immune response. Thus, the presence of relatively large number of B cells that can make RF after nonspecific stimulation provides an explanation for the magnitude of RF production accompanying the secondary immune response.

Variable region sequences of murine IgM anti-IgG monoclonal autoantibodies (rheumatoid factors). A structural explanation for the high frequency of IgM anti-IgG B cells

The nucleotide sequences of heavy and light chains from 10 monoclonal IgM anti-IgG1 (RF) antibodies were determined and reported here as translated amino acid sequences. Only three families of VK light chains were used in these antibodies: VK1 (two examples), VK8 (three examples), and VK19 (four examples). This represents a significant nonrandom selection of light chains. In contrast, all other variable region gene segments (i.e., VH, DH, JH, and JK) were used in a pattern consistent with random selection from the available pool of germline genes. In two cases, the same anti-IgG1 specificity was generated by a combination of very homologous light chains with unrelated heavy chains. We infer from this that the light chain is the segment used by these antibodies to bind IgG1. The nature of these sequences provides an explanation for the curious observation that as many as 15% of splenic B cells in normal mice may be expressing IgM anti-IgG; if, as our data suggest, certain light chains in combination with many different heavy chains can be used in assembling the anti-IgG specificity, then, because of combinatorial association in which the heavy chain is not relevant for specificity, the fraction of IgM-producing B cells expressing these light chains should approximate the fraction of B cells making IgM anti-IgG. We calculate, based on data presented in several other studies, that 5-17% of B cells express one of the VK types observed in monoclonal RF. This agrees well with estimates for the number of B cells making IgM anti-IgG. In addition, our findings could rule out other explanations of the high percentage of B cells making RF, such as constant stimulation by antigen or presence of numerous antigenic epitopes since it was shown that IgM anti-IgG1 antibodies are not somatically mutated and that they are structurally homogeneous. We aligned the VK sequences of the RF in hopes of finding some primary sequence homology between the represented VK families which might point to residues involved in the binding interaction. Although we found no such homology in the hypervariable regions, we did find significant and unexpected homology in the FR2 and FR3 of these light chains. We noted that these regions are exposed in the Ig structure and postulate that they may be involved in a unique type of binding interaction between two Ig family domains, i.e., VK binding to a constant region domain of IgG.

Induction of rheumatoid factors in mice by immune complexes of bacterial lipopolysaccharide with mouse IgG antibody

Administration of 2,4,6-trinitrophenylated E. coli lipopolysaccharide (TNP-LPS) complexed with mouse IgG antibody to TNP specifically gave rise to a marked production of rheumatoid-like factors (RF) in the recipient mice, in contrast to the low and nonspecific RF production via polyclonal B cell activation by the same dosage of LPS or TNP-LPS alone. The RF activity induced by the LPS immune complexes was associated with both IgG and IgM and directed primarily to the C gamma 2 region as judged by the heterophilic reactivity toward fragments of rabbit IgG. The results suggest that antibody molecules attached to LPS constitute novel epitopic groups on the mitogenic carrier and stimulate B cells in a specific manner to induce the autoantibodies.

Rheumatoid factors in 129XB recombinant inbred strains. Igh-1-linked control of allotypic and isotypic specificities

To examine the role of autologous IgG in the induction of murine rheumatoid factors (RF) we have analyzed the allotypic specificity of anti-IgG2a RF in recombinant inbred strains derived from 129/Sv (Igh-1a) and C57BL/6 (Igh-1b) mice. In five of six Igh-1a strains, anti-IgG2a RF reacted with IgG2aa but failed to react with IgG2ab. In contrast, isotype-specific RF, which reacted equally well with a and b allotypes of IgG2a, represented the major RF species in one Igh-1a and all five Igh-1b strains tested. An additional form of RF specific for IgG2ab and not reactive with IgG2aa was detected in one Igh-1b strain. RF specific for a give allotype was thus only found in the presence of that allotype, which strongly suggests the involvement of autologous IgG in the induction of mouse RF synthesis. The specificity of RF was apparently further controlled by genes linked to but different from the Igh-C locus, as indicated by the absence of IgG2aa-specific RF in one of the 6 Igh-1a strains tested. Because this strain, 129XBG, has been shown to express idiotypic markers characteristic of Igh-1b mice, it is likely that the genes, which in the presence of a given allotype induce the production of isotype rather than allotype-specific RF, are identical to those that control the expression of idiotypes. Evidence was also obtained to indicate that Igh-1-linked genes influence the isotypic specificity and the isotype of RF itself: IgA anti-IgG2a predominated in Igh-1a strains and IgM anti-IgG1 in Igh-1b strains. Interestingly enough, total IgA and IgG2a levels also were higher in Igh-1a than in Igh-1b strains.