B and T lymphocytes are the respective origins of adaptive humoral and cellular immunity due to their exquisite Ag-specific receptors. On the other side of the fence, the complement system, neutrophils, and monocytes deal with acute infections through innate mechanisms. This dichotomy permeated the literature until the early 1990s, before a more balanced picture of the overlap and complexities of both immune systems came into focus. After the molecular identification of leukocyte cell surface receptors in the 1980s, B cells were found to only express a handful of lineage-specific molecules. The BCR was historically considered the central fate-determining B cell surface molecule, with the identity of this receptor complex defined in the early 1990s. The costimulatory roles of CD4 and CD8 in T cell Ag receptor signaling were known at that time, with Carter and Fearon (1) suggesting in this 1992 article selected for the Pillars of Immunology series that CD19 fulfilled the equivalent function for B cells. On a larger scale, linking CD19 and its associated CD21 complement receptor with enhanced BCR signaling provided a transforming example of how the innate and adaptive immune systems could overlap functionally and use complement components to influence the outcome of adaptive immune responses.

By 1992, in vitro studies using CD19 mAbs as surrogate ligands had demonstrated that CD19 ligation initiated a series of biological responses of central importance to B cell signaling. Specifically, CD19 cross-linking alone induced an intracellular calcium concentration ([Ca2+]i) response in B cells and some B cell lines (24) and induced tyrosine kinase activity (5, 6). However, CD19 ligation inhibited BCR-induced [Ca2+]i increases and proliferation (24). Although these and multiple other studies indicated that CD19 ligation could negatively regulate B cell signaling, the studies of Carter and Fearon (1) demonstrated that CD19/BCR coligation decreased the threshold for BCR stimulation by two orders of magnitude when a weakly mitogenic anti-IgM mAb was used. Subsequent studies have also demonstrated that CD19 can enhance simultaneous BCR signaling independent of BCR coligation (7, 8). Thus, CD19 can both positively and negatively regulate BCR signaling depending on the relative manner and timing of receptor engagement, with simultaneous CD19 and BCR engagement inducing optimal B cell activation.

CD19 was known to be a central member of a multimeric cell surface signal transduction complex that included CD81 and, when expressed, CD225 (Leu-13) and the CD21 receptor for complement component C3d and EBV (4, 9). The studies of Carter and Fearon (1) along with the previous identification of Lyn as a CD19 binding partner led van Noesel and colleagues (10) to propose that foreign Ags opsonized by complement degradation products could simultaneously engage the BCR and CD21/CD19 complexes. Under these circumstances, Lyn bound to the extensive cytoplasmic domain of tyrosine-phosphorylated CD19 would provide additional tyrosine kinase activity for BCR phosphorylation, with CD19 contributing additional docking sites for molecules involved in BCR signaling. Thereby, extra- and intracellular cooperation between the BCR and CD21/CD19 could provide optimal recognition for monomeric Ags or enhance signaling when BCR affinities were low. Ag-C3d bridging of the innate and adaptive immune responses through the dual engagement of CD21/CD19 and the BCR was subsequently demonstrated (11). Fusing multiple copies of C3d to lysozyme remarkably lowered the dose of lysozyme required for Ab responses in mice by at least 1000-fold. More recently, C3d-conjugation or fusion with multiple copies of C3d has been shown to induce 2- to 8-fold higher Ab responses to a number of physiologically important immunogens, including HIV-gp120, viral hemagglutinin, and pneumococcal polysaccharide (reviewed in Ref. 12). However, C3d also functions as a molecular adjuvant in CD21−/− mice (12). Thereby, C3d bound to Ag can facilitate humoral immunity through both CD21-dependent and -independent pathways.

As with CD19 mAbs, both positive and negative roles for C3d have been demonstrated, where C3d–Ag complexes can either augment or inhibit B cell activation and humoral immunity depending on the degree of CD21/CD19 complex engagement (13, 14). For example, low concentrations of streptavidin-C3dg tetramers dramatically enhance BCR-induced [Ca2+]i responses, with significantly enhanced CD19 and Lyn phosphorylation and CD19/Lyn conjugate formation. Under these conditions, negative regulatory signaling is also abrogated as characterized by reduced BCR-induced CD22 phosphorylation and Src homology region 2 domain-containing phosphatase 1/CD22 associations (7). By contrast, CD19/CD21 ligation using higher streptavidin-C3dg concentrations inhibits BCR-induced [Ca2+]i responses and CD19, Lyn, and Syk phosphorylation. Positive and negative regulatory roles for the CD19/CD21 complex are explained by the extent to which CD19 recruits Lyn and other downstream signaling molecules following BCR engagement (15). The CD19 cytoplasmic domain functions as an adapter molecule for the recruitment and “processive amplification” of Lyn, which significantly augments BCR-induced [Ca2+]i responses (16). Because CD19 and Lyn expression are genetically titrated in B cells, CD19/Lyn complex formation sequesters the available pool of functional Lyn away from downstream negative regulatory proteins (7), including CD22 that normally regulates the extent of CD19 phosphorylation through a negative feedback loop (17). However, when CD19 or CD21 are maximally cross-linked, CD19/Lyn complex formation sequesters Lyn away from both the BCR and CD22 (13, 14). Thereby, CD19 recruitment of Lyn preferentially activates selective signaling pathways to the exclusion of other downstream regulatory and effector pathways. CD21 is also proposed to physically sequester CD19 away from the BCR under some conditions, thereby dampening BCR-mediated signals (18). Thus, the CD19/CD21 complex functions as a general rheostat that defines intrinsic and BCR-induced signaling thresholds critical for B cell activation and clonal expansion.

Altering CD19 density on the cell surface has profound effects on B cell development, subset distributions, activation, proliferation, and humoral immunity (15). Although CD19 can regulate B cell signal transduction independent of CD21 expression and complement activation (19), CD21 deficiency increases cell surface CD19 expression by ∼50% (19, 20), which has significant effects on humoral immunity. For example, impaired humoral immune responses to Streptococcus pneumoniae capsular polysaccharide and other strong T cell-independent type 2 (TI-2) Ags in CD21−/− mice are due to CD19 overexpression (21). Reducing CD19 expression in CD21−/− mice normalizes Ag-specific IgG3 responses to TI-2 Ags and completely restores protection during S. pneumoniae infection. Even so, optimal Ab responses to weak or poorly haptenated TI-2 Ags do require C3 and CD21 expression. Thus, CD21 promotes protective humoral immunity to strong TI-2 Ags through a complement-independent pathway by negatively regulating cell surface CD19 densities, in addition to serving as a receptor for C3 cleavage products. Thereby, CD19 and CD21 function as overlapping molecular rheostats to appropriately balance immune responses.

The studies of Carter and Fearon (1) focused attention on CD19 as an important cell surface receptor that critically regulates Ag-specific B cell responses. The subsequent importance and complexity of CD19 function has increased. CD19 on its own serves as an important modulator of B cell signaling, whereas its ability to combine with cell surface CD21 provides an innate sensing mechanism that allows all B cells to detect activated complement fragments within inflammatory microenvironments regardless of their BCR specificity. These sensitized B cells are thereby primed to generate robust and immediate responses when their BCRs are simultaneously engaged by either Ag or C3d–Ag complexes. Conversely, inflammation-induced B cell hyperactivity due to excessive complement activation and the generation of C3d-immune complexes must be regulated to prevent untoward B cell expansion and pathology, such as in cases of autoimmunity. In this event, the ability of the CD19/CD21 complex to detect high levels of C3d-bearing immune complexes provides a mechanism through which B cells can be desensitized but not deleted. The CD19/CD21 complex thereby functions as a “response regulator” that titrates signals generated through the BCR and other cell surface molecules to balance between protective immunity and autoimmunity. Thereby, the innate and adaptive immune systems are not separable, but are intimately intertwined to regulate immune responses. Our understanding of the multifunctional nature of the CD19/CD21 complex remains worthy of further exploration, which may provide novel opportunities for new vaccines and treatments for autoimmunity.

I thank Drs. Karen Haas, Jonathan Poe, and Evgueni Kountikov for their suggestions.

Disclosures The author has no financial conflicts of interest.

Abbreviations used in this paper:

[Ca2+]i

intracellular calcium concentration

TI-2

T cell-independent type 2.

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