ORIGIN AND PROPERTIES OF SOLUBLE CD21 (CR-2) IN HUMAN BLOOD

By analysis with a panel of CD21 MoAbs it is shown that a large part o f the soluble CD21 in human blood plasma is of the long isoform (CD21L ), as judged by comparison with antigen produced by mouse L cells tran sfected with CD21L-cDNA and reactivity with the restricted CD21 MoAb R 4/23. This is compatible with the hypothesis that soluble CD21 in the blood is mainly derived from follicular dendritic cells (FDC). Cells f rom a human keratinocyte cell line transfected with cDNA from the Burk itt lymphoma cell line Raji also produced soluble CD21L (sCD21L), wher eas the short form of sCD21 (sCD21S) was the major component of sCD21 produced by the B lymphoblastoid cell line LICR-LON-HMy and the T cell line Jurkat. Confocal studies of FDC isolated from human tonsil revea led that CD21 was present in the cytoplasm. On gel filtration sCD21 fr om untreated serum has an apparent size considerably greater than the 130 kD found by SDS-PAGE analysis. This may be partly accounted for by the non-globular shape of the molecule, but may also indicate, as rep orted by others, that in its native state sCD21 is complexed with othe r proteins. However, no evidence of complexing with sCD23 or C3d could be found.