Depending on the stimuli they encounter, B lymphocytes engage in signaling events that lead to immunity or tolerance. Both responses are mediated through antigen interactions with the B cell antigen receptor (BCR). Antigen valency is thought to be an important parameter in B cell signaling, but systematic studies are lacking. To explore this issue, we synthesized multivalent ligands of defined valencies using the ring-opening metathesis polymerization (ROMP). When mice are injected with multivalent antigens generated by ROMP, only those of high valencies elicit antibody production. These results indicate that ligands synthesized by ROMP can activate immune responses in vivo. All of the multivalent antigens tested activate signaling through the BCR. The ability of antigens to cluster the BCR, promote its localization to membrane microdomains, and augment intracellular Ca2+ concentration increases as a function of antigen valency. In contrast, no differences in BCR internalization were detected. Our results indicate that differences in the antigenicity of BCR ligands are related to their ability to elicit increases in intracellular Ca2+ concentration. Finally, we observed that unligated BCRs cluster with BCRs engaged by multivalent ligands, a result that suggests that signals mediated by the BCR are amplified through receptor arrays. Our data suggest a link between the mechanisms underlying signal initiation by receptors that must respond with high sensitivity.

CD22 is an inhibitory coreceptor on the surface of B cells that attenuates B cell antigen receptor (BCR) signaling and, therefore, B cell activation. Elucidating the molecular mechanisms underlying the inhibitory activity of CD22 is complicated by the ubiquity of CD22 ligands. Although antigens can display CD22 ligands, the receptor is known to bind to sialylated glycoproteins on the cell surface. The propinquity of CD22 and cell-surface glycoprotein ligands has led to the conclusion that the inhibitory properties of the receptor are due to cis interactions. Here, we examine the functional consequences of trans interactions by employing sialylated multivalent antigens that can engage both CD22 and the BCR. Exposure of B cells to sialylated antigens results in the inhibition of key steps in BCR signaling. These results reveal that antigens bearing CD22 ligands are powerful suppressors of B cell activation. The ability of sialylated antigens to inhibit BCR signaling through trans CD22 interactions reveals a previously unrecognized role for the Siglec-family of receptors as modulators of immune signaling.B cell antigen receptor ͉ multivalency ͉ sialic acid ͉ siglec ͉ autoimmunity T he initiation of an immune response or the prevention of autoimmunity depends upon the ability of the B cell antigen receptor (BCR) to transmit signals that positively or negatively regulate B lymphocyte survival, proliferation, and differentiation (1). To avoid detrimental autoimmune responses, a means of differentiating between foreign and self-antigens is required; coreceptors that modulate BCR signaling can ensure that these distinctions are made. CD22 is an inhibitory coreceptor that can attenuate BCR signaling (2, 3). CD22 null mice possess hyperresponsive B cells (4), illustrating a role for CD22 in establishing a threshold for B cell activation. Specifically, an increase in intracellular Ca 2ϩ ion concentration is a hallmark of B cell activation (5, 6), and B cells isolated from CD22 null mice display increased Ca 2ϩ flux in response to antigen (4, 7). Thus, loss of CD22 results in a lowering of the threshold for B cell activation. Other data also support this conclusion: CD22 null mice exhibit increased serum IgM concentrations, decreased surface IgM levels on peripheral B cells, increased induction of apoptosis in response to BCR crosslinking, and increased serum autoantibody titers (8). These observations are consistent with the loss of CD22 leading to increased sensitivity and chronic B cell activation.The process of B cell activation ensues upon binding of multivalent antigen to the BCR. Antigen-induced clustering elicits phosphorylation of the cytoplasmic immunoreceptor tyrosinebased activation motifs (ITAMs), which are present in the BCRassociated signaling proteins Ig␣/␤. The phosphorylation reaction is catalyzed by Src-family kinases such as Lyn. Upon phosphorylation of the BCR components, Syk kinase is recruited to the BCR signaling complex (9). Syk is essential for propagating BCR signaling (10, 11). It acts along with...

The applications of block copolymers are myriad, ranging from electronics to functionalized resins to therapeutics. The ring-opening metathesis polymerization (ROMP) is an especially valuable reaction for block copolymer assembly because each block can be generated with length control. We sought to use this polymerization to expand the repertoire of block copolymers by implementing a strategy that involves post-polymerization modification of a backbone bearing selectively reactive groups. To this end, we demonstrate that ROMP can be used to synthesize a block copolymer scaffold that possesses three types of functional groups – a succinimidyl ester, an α-chloroacetamide group, and a ketone – each of which can be modified independently. Thus, a single scaffold can be elaborated to afford a wide range of block copolymers. Exploiting this synthetic approach and the length control offered by ROMP, we assemble block copolymers capable of traversing the membrane and entering mammalian cells.

We report a general method for the solid-phase synthesis of polymers via the ring-opening metathesis polymerization (ROMP). The method involves polymerization in solution to form a block copolymer, immobilization of the polymer via reaction of one block with a resin-bound functional group, modification of the other block, and liberation of the polymer from the resin. We demonstrated the utility of this approach by generating a block copolymer with an N-hydroxysuccinimidyl ester-substituted block (for on-resin functionalization) and a maleimide-substituted block (for conjugation to the resin). We showed that the Diels-Alder reaction can be employed to immobilize the polymers and that amines of diverse structure can be used to modify the resin-bound polymers. The reversibility of the furan-maleimide Diels-Alder adduct was exploited to liberate the polymer from the support. Specifically, treatment of the resin with cyclopentadiene resulted in complete polymer release. The resulting polymers are functional: they were as potent in assays with the lectin concanavalin A as polymers generated by traditional solution routes. We anticipate that this method can be used for the rapid synthesis of diverse polymers via ROMP.

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