Complement factors H and I synthesized by B cell lines function to generate a growth factor activity from C3.
B lymphocytes and transformed B lymphoblastoid cell lines express CR2 (CD21, C3d/EBV-receptor) that is specific for C3 fragments generated by cleavage of C3b or spontaneously hydrolyzed native C3 (C3i) by the serum enzyme factor I and its cofactor, factor H. It had been shown previously that the Raji B cell line could be cultivated in serum-free medium supplemented with only transferrin and either OKB7 anti-CR2 mAb, C3d, or C3d-derived peptides containing the CR2 binding site. Because these agents appeared to function through ligation of CR2, it was unclear how native C3 could also serve as a growth factor, because C3 does not bind to CR2. It appeared possible that Raji cells might be able to use endogenous factors H and I to generate a CR2 ligand from C3, because previous studies had shown that Raji cells synthesized factor H and probably also synthesized factor I. PCR analysis was used to demonstrate factor I mRNA in Raji cells. Secretion of Raji cell factor I protein was confirmed by a sensitive mAb ELISA. Several B cell lines were examined for C3-dependent growth. Raji cells required both C3 (or OKB7) and transferrin for growth, whereas Wil-2 cells grew with transferrin alone and C3 enhanced the growth-promoting activity of transferrin. Two other B cell lines (Daudi and U698M), the T cell line 8402, and the U937 monocytoid cell line could not be sustained with transferrin plus C3. The C3-dependent growth of Raji cells was inhibited almost completely by either OX-23 anti-factor H or 052.11.3 anti-factor I mAb that also blocked the activity of serum-derived factor H or I, respectively. By contrast, there was no inhibition of growth by either OX-24 anti-factor H or OX-21 anti-factor I mAb that did not block factors H and I activity. After the spontaneous hydrolysis of native C3 to C3i, it is hypothesized that Raji cells convert C3i to iC3i with endogenous factors H and I, and then this iC3i serves as a growth factor by binding to membrane CR2.
Vĕtvicka
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Department of Microbiology and Immunology, University of Louisville, KY 40292.
Reed
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Hoover
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Ross
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D000818:Animals; D001402:B-Lymphocytes / Q000502:physiology*; D001483:Base Sequence; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D003176:Complement C3 / Q000378:metabolism* / Q000494:pharmacology; D017242:Complement Factor H / Q000096:biosynthesis / Q000502:physiology*; D017244:Complement Factor I / Q000096:biosynthesis / Q000502:physiology*; D016895:Culture Media, Serum-Free; D006133:Growth Substances / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D014168:Transferrin / Q000494:pharmacology
C493495:CFH protein, human; D003176:Complement C3; D016895:Culture Media, Serum-Free; D006133:Growth Substances; D014168:Transferrin; D017242:Complement Factor H; D017244:Complement Factor I