A human CR1-like transcript containing sequence for a binding protein for iC4 is expressed in hematopoietic and fetal lymphoid tissue.

Primate immune adherence receptors are erythrocyte complement receptors (E-CR) that favorably influence the clearance of circulating immune complexes (IC). The human E-CR is the type one complement receptor (CR1), most commonly expressed as a 220 kDa protein containing 30 short consensus repeats (SCRs). The chimpanzee E-CR is a 75 kDa protein composed of eight SCRs, and is encoded by an ortholog of human CR1-like (CR1L), a genetic element related to CR1. Human CR1L was previously identified from genomic clones that predict exons for seven SCRs, and there have been no reports of CR1L expression. The purpose of this study was to determine if human CR1L is expressed. Amplification of human bone marrow cDNA using primers specific for CR1/CR1L yielded a product similar to chimp CR1L encoding sequence. The first 6.5 SCRs matched 100% with the predicted human CR1L sequence, while the second half of SCR 7 was homologous to the comparable chimp CR1L sequence but with a stop codon. Expression in COS-7 cells yielded a human CR1L protein of approximately 50 kDa that exhibited binding specificity for iC4 but not for iC3. Neither northern nor western blot analysis of human bone marrow revealed the presence of the CR1L transcript or protein. However, northern blot analysis of various other lymphoid tissue identified a candidate CR1L transcript in human fetal liver. PCR amplification of a cDNA panel of human fetal tissue confirmed the presence of the CR1L transcript in fetal liver, and to a lesser extent in fetal spleen and thymus. Thus, expression of the CR1L transcript appears to be limited to hematopoietic and fetal lymphoid tissue.

A CR1 polymorphism associated with constitutive erythrocyte CR1 levels affects binding to C4b but not C3b.

The erythrocyte type one complement receptor (E-CR1) mediates erythrocyte binding of complement-opsonized immune complexes (IC), and helps protect against random deposition of circulating IC. Two linked CR1 polymorphisms occur in binding domains, at I643T and Q981H. In Caucasians, the variant alleles (643T, 981H) are associated with low constitutive E-CR1 expression levels. This study was conducted to determine if these polymorphisms affect ligand binding, and if so, represent risk factors for the autoimmune IC disease, systemic lupus erythematosus (SLE). In an ELISA comparing relative ligand binding differences, E-CR1 from individuals homozygous for the variant residues (643TT/981HH) exhibited greater binding to C4b, but not C3b, than homozygous wild-type E-CR1. Analysis of single-binding domain CR1 constructs demonstrated that the 981H residue imparted this enhanced C4b binding. No differences were observed in the 981H allele frequency between Caucasian controls (0.170, n = 100) and SLE patients (0.130, n = 150, P = 0.133), or between African American controls (0.169, n = 71) and SLE patients (0.157, n = 67). In a subset of individuals assessed for CR1 size, excluding from this analysis those expressing at least one B allele revealed a trend for over-representation of the 981H allele in Caucasian controls (0.231 frequency, n = 26) versus SLE patients (0.139, n = 83, P = 0.089), but again no difference between African American controls (0.188, n = 24) and SLE patients (0.191, n = 34). These data suggest that the 981H residue compensates for low constitutive expression of E-CR1 in Caucasians by enhancing C4b binding. This may contribute protection against SLE.