Identification and characterization of complement C3 receptors on human astrocytes.

Astrocytes express C components and have been implicated as a major source of intrathecal C. To ascertain the effects of C activation on these cells, we have evaluated the expression of CR1, CR2, and CR3 (CD35, CD21, and CD11b/CD18) in primary fetal astrocytes and astrocyte cell lines. None of the astrocyte cells tested expressed CR3, whereas primary astrocytes and one of four astrocyte cell lines expressed CR1 (220 kDa), as assessed at the protein and mRNA level. Primary fetal astrocytes and all four astrocyte cell lines expressed CR2 (155 kDa). Expression of CR2 by astrocytes was confirmed at mRNA level by reverse-transcriptase PCR, using different combinations of seven specific CR2 oligonucleotides, and by partial sequencing of the astrocyte CR2 cDNA. Astrocyte CR2 cDNA presented 100% homology with the lymphocyte CR2 cDNA between the position 181 bp to 600 bp and position 1017 bp to 1347 bp. An alternative splicing pattern of exon 11, reported previously in B cells, was observed in astrocyte CR2 cDNA. Astrocyte CR2 was functional, in that it specifically bound C3d and the EBV surface protein gp340, and the binding was blocked specifically with polyclonal anti-CR2. Scatchard analysis of membrane expression of CR2 on astrocytes revealed 2000 functional sites per cell with a Kd (3 x 10(-7) M) identical with that of CR2 on B cell (Raji).