Linkage relationship of C2 deficiency, HLA and glyoxalase I loci

Facilitation of complement-dependent killing of the Lyme disease spirochete, Borrelia burgdorferi, by specific immunoglobulin G Fab antibody fragments

In the absence of specific antibody, Borrelia burgdorferi is resistant to the bactericidal action of complement, despite the capacity of the spirochete to activate complement. Complement-mediated killing of B. burgdorferi requires the presence of antiborrelial immunoglobulin G (IgG). The effect of bactericidal IgG takes place after formation of the C5 convertase. Therefore, we examined the ability of Fab fragments from bactericidal IgG to mediate killing of B. burgdorferi by complement. The complement-activating domain of IgG, the Fc fragment, was not required for killing of borreliae, as monovalent Fab fragments prepared from immune IgG were also able to mediate killing. However, the killing efficiency of the Fab fragments was less than that of intact IgG, suggesting that the bactericidal activity of IgG is enhanced by divalency. IgG Fab-mediated killing occurred without increased complement activation or C3 fluid-phase consumption. Cell killing proceeded via the classical complement pathway, as no killing of Fab fragment-sensitized cells was observed in human serum deficient in C2. These results demonstrate directly that the bactericidal effect of anti-B. burgdorferi IgG is independent of the complement-activating properties of the antibody.

Release of heparan sulfate from endothelial cells. Implications for pathogenesis of hyperacute rejection

Heparan sulfate proteoglycan associated with endothelial cells in normal blood vessels inhibits intravascular coagulation and egress of blood cells and plasma proteins, key features of hyperacute rejection. It was shown herein that exposure of cultured porcine endothelium to human serum as a source of natural antibodies and complement caused cleavage and release of 5% of endothelial cell proteoglycans within 4 min and greater than 50% within 1 h. Proteoglycan release depended on activation of the classical complement pathway and preceded irreversible cell injury. These findings suggest that loss of endothelial cell proteoglycan may be a critical step in the pathogenesis of hyperacute rejection and in diseases involving humoral injury to endothelial cells.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Sex influences transmission of the supratype associated with the C2 deficiency allele: a possible mechanism for the maintenance of heterozygosity and disease susceptibility

We have examined the transmission of the supratype associated with C2 deficiency (C2Q0) using our own and many published pedigrees in order to determine whether nonrandom transmission may contribute to the stability of disease-associated supratypes. Unexpectedly, we found that C2Q0 may be preferentially transmitted to the opposite sex and therefore propose that such "zig-zag" transmission may result in the preservation of recessive disease susceptibility genes and in the maintenance of heterozygosity. Further pedigrees are required to verify our findings.

Complement loci of the HLA complex. Studies on families with intra-HLA crossovers and haplotype associations

We report genetic studies of families with defined crossovers in the HLA complex. Haplotypic associations between the different alleles have been analyzed. The object of the studies has been to determine the precise location of complement loci on the HLA complex on chromosome 6. Based on direct observation of recombinations and on indirect evidence from haplotypes, we postulate that the C4 and Bf loci are located between the HLA-B and -D regions and probably closer to the former. Available information also points towards the same localization for the C2 locus.

The location of C2, C4, and BF relative to HLA-B and HLA-D

The loci for HLA-A,B,C,D, and DR are known to be closely linked to the structural loci for the complement components C2, BF, and the duplicated loci for C4, C4A and C4B. Conflicting evidence has been presented for the order of these genes. However, new techniques have made possible identification of markers in the HLA-D and C4 region for nearly all identified haplotypes. In our population we have confirmed five HLA-B-D crossovers and in each case informative allotypes of C2, BF, or C4A and C4B segregated with HLA-D or DR suggesting that the loci for these proteins lie close to HLA-D and DR. These findings may be of importance for resolving problems encountered in the assignment of HLA-D alleles.

On the significance of C2, C4, and factor B polymorphisms in disease

In this review article, recent evidence is presented that some diseases like insulin-dependent diabetes mellitus, multiple sclerosis, and idiopathic membranous nephropathy, which are primarily associated with HLA-D,DR, are also related to the rare C2, C4, and Factor B alleles. Circumstantial evidence is available that at least some of these rare variants may be functionally deficient. Based on the concept of functionally interacting gene clusters, mutant complement genes may lead to impaired effector mechanisms in virus neutralization or lysis of virus-infected cells. Other mechanisms such as alteration of vascular permeability may be involved in the development of proliferative retinopathy and familial hypertension. In lepromatous lepra, an impaired cell-mediated lysis of M. leprae may be related to the hemolytically inactive C4F1 allelic product.