Complement component C3d-antigen complexes can either augment or inhibit B lymphocyte activation and humoral immunity in mice depending on the degree of CD21/CD19 complex engagement.
C3d can function as a molecular adjuvant by binding CD21 and thereby enhancing B cell activation and humoral immune responses. However, recent studies suggest both positive and negative roles for C3d and the CD19/CD21 signaling complex in regulating humoral immunity. To address whether signaling through the CD19/CD21 complex can negatively regulate B cell function when engaged by physiological ligands, diphtheria toxin (DT)-C3d fusion protein and C3dg-streptavidin (SA) complexes were used to assess the role of CD21 during BCR-induced activation and in vivo immune responses. Immunization of mice with DT-C3d3 significantly reduced DT-specific Ab responses independently of CD21 expression or signaling. By contrast, SA-C3dg tetramers dramatically enhanced anti-SA responses when used at low doses, whereas 10-fold higher doses did not augment immune responses, except in CD21/35-deficient mice. Likewise, SA-C3dg (1 microg/ml) dramatically enhanced BCR-induced intracellular calcium concentration ([Ca2+]i) responses in vitro, but had no effect or inhibited [Ca2+]i responses when used at 10- to 50-fold higher concentrations. SA-C3dg enhancement of BCR-induced [Ca2+]i responses required CD21 and CD19 expression and resulted in significantly enhanced CD19 and Lyn phosphorylation, with enhanced Lyn/CD19 associations. BCR-induced CD22 phosphorylation and Src homology 2 domain-containing protein tyrosine phosphatase-1/CD22 associations were also reduced, suggesting abrogation of negative regulatory signaling. By contrast, CD19/CD21 ligation using higher concentrations of SA-C3dg significantly inhibited BCR-induced [Ca2+]i responses and inhibited CD19, Lyn, CD22, and Syk phosphorylation. Therefore, C3d may enhance or inhibit Ag-specific humoral immune responses through both CD21-dependent and -independent mechanisms depending on the concentration and nature of the Ag-C3d complexes.
Lee
Youngkyun
Y
Department of Immunology, Duke University Medical Center, Durham, NC 27710, USA.
Haas
Karen M
KM
Gor
Dennis O
DO
Ding
Xuedong
X
Karp
David R
DR
Greenspan
Neil S
NS
Poe
Jonathan C
JC
Tedder
Thomas F
TF
D000818:Animals; D000917:Antibody Formation; D000918:Antibody Specificity / Q000276:immunology; D000941:Antigens / Q000378:metabolism; D018941:Antigens, CD19 / Q000276:immunology / Q000378:metabolism*; D001402:B-Lymphocytes / Q000276:immunology*; D015933:Complement C3d / Q000276:immunology* / Q000378:metabolism; D008213:Lymphocyte Activation / Q000276:immunology*; D051379:Mice; D018345:Mice, Knockout; D011485:Protein Binding / Q000276:immunology; D011947:Receptors, Antigen, B-Cell / Q000276:immunology; D017464:Receptors, Complement 3d / Q000276:immunology / Q000378:metabolism*; D015398:Signal Transduction / Q000276:immunology
D000941:Antigens; D018941:Antigens, CD19; D011947:Receptors, Antigen, B-Cell; D017464:Receptors, Complement 3d; D015933:Complement C3d