IDENTIFICATION AND CHARACTERIZATION OF COMPLEMENT C3 RECEPTORS ON HUMAN ASTROCYTES

Astrocytes express C components and have been implicated as a major so urce of intrathecal C. To ascertain the effects of C activation on the se cells, we have evaluated the expression of CR1, CR2, and CR3 (CD35, CD21, and CD11b/CD18) in primary human 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 a strocytes and all four astrocyte cell lines expressed CR2 (155 kDa). E xpression 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 cD NA 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 functi onal, in that it specifically bound C3d and the EBV surface protein gp 340, and the binding was blocked specifically with polyclonal anti-CR2 . Scatchard analysis of membrane expression of CR2 on astrocytes revea led 2000 functional sites per cell with a K-d (3 x 10(-7) M) identical with that of CR2 on B cell (Raji).