Human lymphocytes shed a soluble form of CD21 (the C3dg/Epstein‐Barr virus receptor, CR2) that binds iC3b and CD23

Abstract

We report on a soluble (s) form of CD21 (the C3dg/Epstein-Barr virus receptor, CR2) that is spontaneously released by B and T lymphocytes. Immunoprecipitation with anti-CD21 mAb of culture supernatants of surface and biosynthetically labeled B and T cell lines revealed a single band with an apparent molecular mass of 135 kDa. The molecule exhibited a molecular mass 10 kDa lower than that of membrane CD21. The release of soluble CD21 (sCD21) was time dependent and correlated with a parallel decrease in the expression of the membrane-associated molecule. The protein was also found in culture supernatants of tonsillar B cells and normal human thymocytes. Epitopic analysis using combinations of anti-CD21 monoclonal antibodies (mAb) indicated that sCD21 and membrane CD21 were similarly recognized by mAb directed against short consensus repeats (SCR) 1-2, SCR 4-5 and SCR 9-11. Affinity-purified sCD21 was capable of binding to purified human iC3b and to human recombinant CD23, as assessed by enzyme-linked immunosorbent assay and by using the BIAcore technology. In addition, normal human serum was found to contain a soluble form of CD21 that exhibited a similar molecular mass to that of the molecule shed by B and T cells in culture. The serum form of CD21 was recognized by all anti-CD21 mAb that we tested and showed a high reactivity with mAb directed against SCR 1-2. Our observations suggest that B and T cells shed the extracellular portion of CD21 and release a soluble molecule that retains the ligand-binding properties of CD21, thus having a potential role in immunoregulation.

Contribution of Follicular Dendritic Cells to Persistent HIV Viremia

“…Following the work of Hlavacek et al (26), the average CR2 density is ϽR T Ͼ ϭ 9.6 ϫ 10 9 cm Ϫ2 (28)(29)(30); the number of sites available on a virion to bind CR2 is estimated to be between 10 and 100 (31-33), and we explore using an average value ϽnϾ ϭ 20. The estimates for the dissociation rate constants, k r ϭ k Ϫx ϭ 0.1 s Ϫ1 (34), and the attachment rate constant, ␣ ϭ 3.6 ϫ 10 Ϫ11 ml s Ϫ1 , are also taken from the work of Hlavacek et al (26). We choose the surface association constant, i.e., the cross-linking rate constant k x , to be 10 Ϫ12 Ϫ 10 Ϫ11 cm 2 s Ϫ1 , which is in the range of values estimated by Hlavacek et al (26,(34)(35)(36)(37).…”

“…The estimates for the dissociation rate constants, k r ϭ k Ϫx ϭ 0.1 s Ϫ1 (34), and the attachment rate constant, ␣ ϭ 3.6 ϫ 10 Ϫ11 ml s Ϫ1 , are also taken from the work of Hlavacek et al (26). We choose the surface association constant, i.e., the cross-linking rate constant k x , to be 10 Ϫ12 Ϫ 10 Ϫ11 cm 2 s Ϫ1 , which is in the range of values estimated by Hlavacek et al (26,(34)(35)(36)(37). By trying different values of k x , we find that the viremia data in the work of Palmer et al (8) and Maldarelli et al (38) can be simultaneously fitted using k x ϭ 0.38 k Ϫx /ϽR T Ͼ, i.e., k x ϭ 3.96 ϫ 10 Ϫ12 cm 2 /s.…”

Expression of complement receptors 1 (CR1/CD35) and 2 (CR2/CD21), and co-signaling molecule CD19 in cattle

“…Soluble CR2 (sCR2) has been found in serum and plasma from healthy humans (Douglas et al, 2009;Ling et al, 1998;Masilamani et al, 2003;Masilamani et al, 2004a); decreased plasma concentration of sCR2 has been associated with rheumatoid arthritis, SLE, Sjögren's syndrome and pregnancy (Masilamani et al, 2004a;Masilamani et al, 2004b;Masilamani et al, 2008). sCR2 is not a product of an alternatively spliced mRNA transcript that lacks the transmembrane domain (Illges et al, 1997); it is derived from cleavage of the membrane protein's extracellular domains (Frémeaux--Bacchi et al, 1996;Masilamani et al, 2003) through proteolysis by matrix metalloproteases (Aichem et al, 2006). The cleavage site is directly adjacent to the membrane (Illges et al, 1997;Data from Masilamani et al, 2002 andMasilamani et al, 2003 as interpreted by Aichem et al, 2006) and is dependent on cysteine reduction and the opening of a disulphide bridge in the membrane proximal CCP domain to create a juxtamembrane stalk (Aichem et al, 2006).…”

“…sCR2 is capable of binding iC3b, CD23 (Aubry et al, 1992;Frémeaux--Bacchi et al, 1996), IFN--α , and Epstein Barr Virus (EBV) (Moore et al, 1991;Nemerow et al, 1990). Binding sCR2 can lead to changes in gene expression.…”

Expression of CD21 and CD23 during Human Fetal Development

“…The congruency of increased surface CD21 on the T cells and reduced circulating levels makes it tempting to speculate that the difference reflects changes in the pattern of cleavage of membrane CD21 in neonatal compared with adult T cells. Soluble CD21 is formed by protease-mediated cleavage of the extracellular portion of membrane CD21, and in vitro studies reveal a parallel decrease in surface expression with increasing levels in supernatants (24).…”