Chromosome 6: linkage of the eighth component of complement (C8) to the histocompatibility region (HLA)

Inherited deficiency of C8 in a patient with recurrent meningococcal infections: further evidence for a dysfunctional C8 molecule and nonlinkage to the HLA system

An adult male with recurrent meningococcal infections is reported whose serum lacked functional C8 activity but possessed antigenic C8. The addition of 1500 U of purified C8/ml of serum restored hemolytic activity to normal. Four to five times more C8 was required to restore bactericidal activity than to restore hemolytic activity. Bactericidal activity could also be restored by mixing the patient's serum with a second C8-deficient serum that lacked detectable antigenic or functional C8. The patient's serum contained bactericidal antibody for groups A, B, C, and Y meningococci and specific antibody to group Y capsular polysaccharide. There was two to three times more bactericidal antibody activity in the serum than in a pool of normal sera for the infecting strain. Family studies disclosed a sibling who was HLA identical to the patient but whose serum contained normal amounts of total hemolytic and C8 functional activity.

HLA-linked C2 deficiency in a Dutch patient with systemic lupus erythematosus

A patient with systemic lupus erythematosus was studied whose blood serum on repeated occasions showed undetectable levels of haemolytic omplement (C). A detailed investigation of individual C components in the serum of the proposita and her family revealed the absence of functional C2 in the patient and half-normal values in the relatives. C4 levels in the family, but not in the patient, were above normal, whereas the levels of factor B were low in all cases. No abnormalities were noted in C3, C9, or C1INH. Tissue typing showed linkage of the C2-deficiency gene with the HLA-A10/B18 and A9/B18 haplotypes. No linkage with red cell antigens and no relationship with plasma kallikrein levels was found.

Linkage studies with C6

The common structural variants of C6 have been used to study the linkage relations of the locus in human pedigrees. Linkage between C6 and RH, Fy, ACP1, MNSS, Jk, HLA, ABO, ESD, Hp, ADA, GPT, Gc, Pi, Gm and Km has been excluded at recombination fractions at least as great as 0-1 in the male.

Absence of the eighth component of complement in association with systemic lupus erythematosus-like disease

A 56-yr-old black woman with absence of the eighth component of complement and a disease compatible with systemic lupus erythematosus is described. Her disease is characterized by the presence of photosensitive "malar" rash, alopecia, polyarthritis, and nephrotic syndrome. Hemolytic and immunochemical assays of the serum complement components were normal, except for C8 which was undetectable. Hemolytic activity could be restored to normal by the addition of functionally pure C8. In vitro tests to investigate the functional integrity of the classical and alternative pathways indicated that the functions mediated by activation of C3 and C5 were intact whereas heatlabile bactericidal activity was totally absent. Addition of C8 restored this function to normal levels. One of two brothers of the proband was shown to have serum C8 levels approaching 50% of normal indicating the hereditary nature of the defect. HLA typing studies showed that the normal brother had identical haplotypes to the proband while the proposed heterozygous brother only shared one haplotype with the patient, suggesting that the gene controlling the C8 defect was not closely linked to the major histocompatibility complex. If the association of a connective tissue disease and absence of a terminal component of complement is not coincidental, these results suggest that C8 deficiency may be associated with a subtle defect in the defense mechanisms to viral infection leading to viral persistance and perhaps to diseases such as systemic lupus erythematosus where chronic viral infections have been implicated.