Discovery and Preclinical Characterization of BIIB091, a Reversible, Selective BTK Inhibitor for the Treatment of Multiple Sclerosis

Multiple Sclerosis is a chronic autoimmune neurodegenerative disorder of the central nervous system (CNS) that is characterized by inflammation, demyelination, and axonal injury leading to permeant disability. In the early stage of MS, inflammation is the primary driver of the disease progression. There remains an unmet need to develop high efficacy therapies with superior safety profiles to prevent the inflammation processes leading to disability. Herein, we describe the discovery of BIIB091, a structurally distinct orthosteric ATP competitive, reversible inhibitor that binds the BTK protein in a DFG-in confirmation designed to sequester Tyr-551, an important phosphorylation site on BTK, into an inactive conformation with excellent affinity. Preclinical studies demonstrated BIB091 to be a high potency molecule with good drug-like properties and a safety/tolerability profile suitable for clinical development as a highly selective, reversible BTKi for treating autoimmune diseases such as MS.

Generation of PT101, a highly selective IL-2 mutein for treatment of autoimmune diseases

Regulatory T cells (Treg) are vital for immune homeostasis and are dysfunctional in autoimmunity. Interleukin 2 (IL-2) drives the proliferation and function of Tregs via its heterotrimeric receptor (CD25/CD122/CD132). Low dose IL-2 is being evaluated for treatment of autoimmune diseases and has been shown to expand Tregs, yet it has a small selectivity window over conventional T cells (Tconv) and natural killer (NK) cells. To enhance IL-2 selectivity, mutations can be introduced to reduce its CD122/CD132 affinity thus creating a CD25 dependency for signaling through CD122/CD132 upon IL-2 facilitated CD25/CD122/CD132 trimer formation. Using structure guided approach, we introduced mutations in IL-2 that significantly decreased CD122 affinity in addition to mutations that increased CD25 affinity. Finally, we explored other mutations, orientation, and linkers to generate a potent, selective molecule with drug-like manufacturability. These structure activity relationship efforts culminated in the generation of PT101, a mutant IL-2 Fc fusion that was selective in activating and expanding Tregs. PT101 selectively induced STAT5 phosphorylation in human and non-human primate (NHP) Tregs in vitro. In humanized mice and NHPs, administration of PT101 dose-dependently and selectively expanded Treg without significant effects on other immune cell types, nor eliciting proinflammatory cytokine production. In a Phase 1a clinical trial, PT101 was well-tolerated and selectively expanded total Tregs by up to a mean maximum of 3.6-fold. There was no evidence of expansion of NK cells nor pro-inflammatory Tconv at any dose studied. Clinical trials in patients with ulcerative colitis and systemic lupus erythematosus are planned with PT101.

Tissue-specific complement inhibition by antibody-Complement Regulatory Protein (CRP) fusions utilizing Therapeutic Autoimmune reguLatOry proteiN (TALON™) platform

The complement system plays a major role in humoral innate immunity. Upon activation, complement proteins trigger a cascade of enzymatic events leading to the production of inflammatory mediators and membrane attack complexes. Tight regulation of complement proteins is essential for immune homeostasis due to their potency and abundance in fluid phase. Complement Regulatory Proteins (CRPs) that are membrane-bound (e.g., CD59), or in fluid phase (e.g., Factor H), prevent aberrant activation of complement pathways against self and mitigate autoimmunity. The importance of CRPs is underscored by the finding that genetic variations in the CRPs can lead to autoimmune diseases such as paroxysmal nocturnal hemoglobinuria (PNH) characterized by the lysis of host red blood cells from dysregulated complement system.

Systemic complement inhibition via anti-C5 antibody has clinically been shown to benefit some patients with complement-mediated disorders, but the therapy requires a high, frequent dosing scheme and supplemental vaccinations to compensate for systemic immunosuppression. In addition, other diseases such as C3 glomerulopathy affect specific tissues and involve pathways beyond C5, limiting treatment options. To this end, local immunomodulation through targeted delivery of CRPs to specific tissues is an attractive strategy to restore homeostasis in complement-mediated autoimmune diseases while circumventing the need for high dose systemic inhibition of complement factors. Here, we present a proof-of-concept study for the design, generation, and evaluation of tissue-tethering antibodies fused with CRPs. Also, we demonstrate the pharmacokinetics and pharmacodynamics of tissue-targeting CRP fusions in mouse models.

Selective expansion of regulatory T cells by kidney-tethered IL2 mutein

Regulatory T cells (Treg) play a critical role in maintaining graft tolerance following organ transplantation, and increased numbers of Tregs in solid-tissue grafts such as the kidney are associated with improved graft survival and function. Therapeutic approaches that increase Tregs offer a promising alternative to current standard-of-care treatment consisting of broad-acting immunosuppressants and their attendant side effects. While efforts to expand Tregs ex vivo for infusion into patients have shown promise, they present manufacturing and administration challenges. Low dose (LD) interleukin 2 (IL-2) drives Treg proliferation and function via the heterotrimeric IL-2 receptor expressed on Tregs and has been shown to expand Tregs in vivo, including in the context of transplantation. However, IL-2 can also activate other immune cells including conventional T cells and Natural Killer (NK) cells, which express IL-2Rb/CD122 and IL-2g/CD132, and this could accelerate rejection. We have enhanced IL-2 selectivity for Treg by introducing mutations that increase affinity for CD25 and decrease affinity for CD122/CD132, resulting in Treg specific expansion. Here, we demonstrate that this IL-2 Mutein (IL-2M) can be “tethered” to the kidney via a bifunctional antibody targeting a tubule-specific protein. This kidney-tethered IL-2M maintains Treg selectivity and localizes to the kidney with extended tissue PK. Moreover, in a CD34-NSG mouse model, animals dosed with kidney-tethered IL-2M exhibited prolonged kidney Treg expansion compared to a systemic, non-tethered IL-2M. This approach has the potential to substantially improve kidney graft acceptance while reducing adverse effects and improving patient quality of life.

Treatment of Autoimmune and Inflammatory Skin Diseases Using Skin-Targeting Bifunctional Antibodies: A Localized Immunomodulation Approach

Current treatment approaches for autoimmune conditions comprise primarily of systemic immunosuppressants or cytokine blockade. Concentration of therapeutic molecules to the tissues that are the sites of autoimmune and inflammatory diseases is a promising approach with the potential to induce therapeutic benefit and avert risks associated with systemic immunotherapies.

Pandion Therapeutics is developing a bifunctional antibody platform that can drive localized immune modulation. The platform combines a “tether antibody” that targets a tissue of choice and “an effector end” that activates specific regulatory immune pathways to restore immune-homeostasis.

Here we report the engineering of a skin-tethered PD-1 agonist and a skin-tethered CD39 that inhibit T cell activation and function and deplete local ATP, respectively, modulating different arms of the immune system in a tissue specific manner. These skin-tethered immune effectors were assessed for drug-like properties in biophysical assays and in vitro and in vivo assays for target binding, cellular activity and tissue specific-localization. Moreover, these bifunctionals were tested in pathway-relevant preclinical models such as Vitiligo and Contact Hypersensitivity. Strikingly, a selective accumulation of the tethered bifunctionals to the skin was observed and correlated with a tether-dependent efficacy compared to a non-tether control.

We believe that this therapeutic approach has the potential to drive the resolution of cutaneous inflammation, providing an opportunity for developing new targeted therapies for autoimmune and inflammatory skin diseases.

HLA-Restriction of Human Treg Cells Is Not Required for Therapeutic Efficacy of Low-Dose IL-2 in Humanized Mice

Regulatory T (T_reg) cells are essential to maintain immune homeostasis in the intestine and T_reg cell dysfunction is associated with several inflammatory and autoimmune disorders including inflammatory bowel disease (IBD). Efforts using low-dose (LD) interleukin-2 (IL-2) to expand autologous T_reg cells show therapeutic efficacy for several inflammatory conditions. Whether LD IL-2 is an effective strategy for treating patients with IBD is unknown. Recently, we demonstrated that LD IL-2 was protective against experimental colitis in immune humanized mice in which human CD4⁺ T cells were restricted to human leukocyte antigen (HLA). Whether HLA restriction is required for human T_reg cells to ameliorate colitis following LD IL-2 therapy has not been demonstrated. Here, we show that treatment with LD IL-2 reduced 2,4,6-trinitrobenzensulfonic acid (TNBS) colitis severity in NOD.Prkdc^scidIl2rg^-/- (NSG) mice reconstituted with human CD34⁺ hematopoietic stem cells. These data demonstrate the utility of standard immune humanized NSG mice as a pre-clinical model system to evaluate therapeutics targeting human T_reg cells to treat IBD.

Discovery of BIIB068: A Selective, Potent, Reversible Bruton's Tyrosine Kinase Inhibitor as an Orally Efficacious Agent for Autoimmune Diseases

Autoreactive B cell-derived antibodies form immune complexes that likely play a pathogenic role in autoimmune diseases. In systemic lupus erythematosus (SLE), these antibodies bind Fc receptors on myeloid cells and induce proinflammatory cytokine production by monocytes and NETosis by neutrophils. Bruton's tyrosine kinase (BTK) is a non-receptor tyrosine kinase that signals downstream of Fc receptors and plays a transduction role in antibody expression following B cell activation. Given the roles of BTK in both the production and sensing of autoreactive antibodies, inhibitors of BTK kinase activity may provide therapeutic value to patients suffering from autoantibody-driven immune disorders. Starting from an in-house proprietary screening hit followed by structure-based rational design, we have identified a potent, reversible BTK inhibitor, BIIB068 (1), which demonstrated good kinome selectivity with good overall drug-like properties for oral dosing, was well tolerated across preclinical species at pharmacologically relevant doses with good ADME properties, and achieved >90% inhibition of BTK phosphorylation (pBTK) in humans.

PT101: A Treg selective agonist IL-2 mutein therapy for autoimmunity

Regulatory T cells (Treg) play a critical role in immune homeostasis and are dysfunctional in many autoimmune diseases. Interleukin 2 (IL-2), via the heterotrimeric IL-2 receptor (CD25/CD122/CD132), drives the proliferation and function of Treg. CD25 loss-of-function in mice is associated with Treg deficiency and widespread autoimmunity. Low dose IL-2 expands Tregs and is being evaluated therapeutically, however it has a narrow selectivity window over conventional T cells and Natural Killer cells which is dose limiting. To enhance IL-2 selectivity for activating and expanding Treg, mutations can be introduced that create a dependency on CD25 binding for signaling through CD122/CD132 upon IL-2 facilitated CD25/CD122/CD132 trimer formation. We created PT101, a mutant IL-2 Fc fusion, containing a N88D mutation that significantly decreases CD122 binding affinity in addition to other mutations that increase CD25 binding affinity and introduce improved molecular stability and drug-like properties. We found that PT101 selectively induces STAT5 phosphorylation in human and cynomolgus monkey Tregs in vitro. In humanized NOD-scid IL-2Rg-null (NSG) mice and cynomolgus monkeys, single or multiple dose administration of PT101 dose-dependently and selectively expands Treg without significant effects on other immune cell types, and without eliciting proinflammatory cytokine production. A 1-month toxicity study in cynomolgus monkeys indicated that PT101 was safe and well-tolerated at 300× the minimum efficacious dose whilst maintaining its Treg selectivity. PT101 is being evaluated in a randomized, placebo-controlled, double-blind, single ascending dose-escalation trial in healthy volunteers.

BCR-dependent lineage plasticity in mature B cells

B2 cells engage in classical antibody responses, whereas B1 cells are considered carriers of innate immunity, biased toward recognizing epitopes present on the surfaces of common pathogens and self antigens. To explore the role of B cell antigen receptor (BCR) specificity in driving B1 cell differentiation, we developed a transgenic system allowing us to change BCR specificity in B cells in an inducible and programmed manner. Mature B2 cells differentiated into bona fide B1 cells upon acquisition of a B1 cell-typical self-reactive BCR through a phase of proliferative expansion. Thus, B2 cells have B1 cell differentiation potential in addition to their classical capacity to differentiate into memory and plasma cells, and B1 differentiation can be instructed by BCR-mediated self-reactivity, in the absence of B1-lineage precommitment.

Optimization of novel reversible Bruton's tyrosine kinase inhibitors identified using Tethering-fragment-based screens

Since the approval of ibrutinib for the treatment of B-cell malignancies in 2012, numerous clinical trials have been reported using covalent inhibitors to target Bruton's tyrosine kinase (BTK) for oncology indications. However, a formidable challenge for the pharmaceutical industry has been the identification of reversible, selective, potent molecules for inhibition of BTK. Herein, we report application of Tethering-fragment-based screens to identify low molecular weight fragments which were further optimized to improve on-target potency and ADME properties leading to the discovery of reversible, selective, potent BTK inhibitors suitable for pre-clinical proof-of-concept studies.

CD62L is not a reliable biomarker for predicting PML risk in natalizumab-treated R-MS patients

Objective:

To assess if the percentage of CD3⁺CD4⁺CD62L⁺ cells in cryopreserved peripheral blood mononuclear cells (PBMCs) (here termed %CD62L) can predict risk of developing progressive multifocal leukoencephalopathy (PML) and better inform the physician for benefit–risk assessment of natalizumab treatment decisions in a global setting.

Methods:

Cryopreserved PBMCs from 21 natalizumab-treated patients who developed PML and 104 matched natalizumab-treated patients with multiple sclerosis (MS) without PML collected as a part of Biogen clinical trials were retrospectively examined for CD3, CD4, CCR7, CD45RA, and CD62L by flow cytometry.

Results:

In this cohort, %CD62L in natalizumab-treated patients did not predict PML risk. Natalizumab-treated patients with MS without PML showed highly variable %CD62L upon serial sampling. In the STRATA study, the distribution of %CD62L in samples collected more than 6 months before a PML diagnosis, at diagnosis, and in natalizumab-treated patients without PML overlapped. No statistical threshold for risk could be determined. In addition, we demonstrated that lymphocyte viability strongly affects %CD62L, supporting previous reports that %CD62L is inherently unstable following cryopreservation and is sensitive to sample collection.

Conclusion:

Data from this well-controlled cohort of natalizumab-treated patients indicate that %CD62L is not a biomarker of PML risk.

Anti-BDCA2 monoclonal antibody inhibits plasmacytoid dendritic cell activation through Fc-dependent and Fc-independent mechanisms

Type I interferons (IFN-I) are implicated in the pathogenesis of systemic lupus erythematosus (SLE). In SLE, immune complexes bind to the CD32a (FcγRIIa) receptor on the surface of plasmacytoid dendritic cells (pDCs) and stimulate the secretion of IFN-I from pDCs. BDCA2 is a pDC-specific receptor that, when engaged, inhibits the production of IFN-I in human pDCs. BDCA2 engagement, therefore, represents an attractive therapeutic target for inhibiting pDC-derived IFN-I and may be an effective therapy for the treatment of SLE. In this study, we show that 24F4A, a humanized monoclonal antibody (mAb) against BDCA2, engages BDCA2 and leads to its internalization and the consequent inhibition of TLR-induced IFN-I by pDCs in vitro using blood from both healthy and SLE donors. These effects were confirmed in vivo using a single injection of 24F4A in cynomolgus monkeys. 24F4A also inhibited pDC activation by SLE-associated immune complexes (IC). In addition to the inhibitory effect of 24F4A through engagement of BDCA2, the Fc region of 24F4A was critical for potent inhibition of IC-induced IFN-I production through internalization of CD32a. This study highlights the novel therapeutic potential of an effector-competent anti-BDCA2 mAb that demonstrates a dual mechanism to dampen pDC responses for enhanced clinical efficacy in SLE.

The actin and tetraspanin networks organize receptor nanoclusters to regulate B cell receptor-mediated signaling

A key role is emerging for the cytoskeleton in coordinating receptor signaling, although the underlying molecular requirements remain unclear. Here we show that cytoskeleton disruption triggered signaling requiring not only the B cell receptor (BCR), but also the coreceptor CD19 and tetraspanin CD81, thus providing a mechanism for signal amplification upon surface-bound antigen stimulation. By using superresolution microscopy, we demonstrated that endogenous IgM, IgD, and CD19 exhibited distinct nanoscale organization within the plasma membrane of primary B cells. Upon stimulation, we detect a local convergence of receptors, although their global organization was not dramatically altered. Thus, we postulate that cytoskeleton reorganization releases BCR nanoclusters, which can interact with CD19 held in place by the tetraspanin network. These results not only suggest that receptor compartmentalization regulates antigen-induced activation but also imply a potential role for CD19 in mediating ligand-independent "tonic" BCR signaling necessary for B cell survival.

Cytoplasmic Ig alpha serine/threonines fine-tune Ig alpha tyrosine phosphorylation and limit bone marrow plasma cell formation

Igα serine 191 and 197 and threonine 203, which are located in proximity of the Igα ITAM, dampen Igα ITAM tyrosine phosphorylation. In this study, we show that mice with targeted mutations of Igα S191, 197, and T203 displayed elevated serum IgG2c and IgG2b concentrations and had elevated numbers of IgG2c- and IgG2b-secreting cells in the bone marrow. BCR-induced Igα tyrosine phosphorylation was slightly increased in splenic B cells. Our results suggest that Igα serine/threonines limit formation of IgG2c- and IgG2b-secreting bone marrow plasma cells, possibly by fine-tuning Igα tyrosine-mediated BCR signaling.

PI3 kinase signals BCR-dependent mature B cell survival

Previous work has shown that mature B cells depend upon survival signals delivered to the cells by their antigen receptor (BCR). To identify the molecular nature of this survival signal, we have developed a genetic approach in which ablation of the BCR is combined with the activation of specific, BCR dependent signaling cascades in mature B cells in vivo. Using this system, we provide evidence that the survival of BCR deficient mature B cells can be rescued by a single signaling pathway downstream of the BCR, namely PI3K signaling, with the FOXO1 transcription factor playing a central role.

Generation of Gut-Homing IgA-Secreting B Cells by Intestinal Dendritic Cells

Normal intestinal mucosa contains abundant immunoglobulin A (IgA)-secreting cells, which are generated from B cells in gut-associated lymphoid tissues (GALT). We show that dendritic cells (DC) from GALT induce T cell-independent expression of IgA and gut-homing receptors on B cells. GALT-DC-derived retinoic acid (RA) alone conferred gut tropism but could not promote IgA secretion. However, RA potently synergized with GALT-DC-derived interleukin-6 (IL-6) or IL-5 to induce IgA secretion. Consequently, mice deficient in the RA precursor vitamin A lacked IgA-secreting cells in the small intestine. Thus, GALT-DC shape mucosal immunity by modulating B cell migration and effector activity through synergistically acting mediators.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Unidirectional Cre-mediated genetic inversion in mice using the mutant loxP pair lox66/lox71

The Cre/loxP recombination system is a commonly used tool to alter the mouse genome in a conditional manner by deletion or inversion of loxP-flanked DNA segments. While Cre-mediated deletion is essentially unidirectional, inversion is reversible and therefore does not allow the stable alteration of gene function in cells that constitutively express Cre. Site-directed mutagenesis yielded a pair of asymmetric loxP sites (lox66 and lox71) that display a favorable forward reaction equilibrium. Here, we demonstrate that lox66/lox71 mediates efficient and predominantly unidirectional inversion of a switch substrate targeted to the mouse genome in combination with either inducible or cell type-specific cre-transgenes in vivo.

B cell receptor signal strength determines B cell fate

B cell receptor (BCR)-mediated antigen recognition is thought to regulate B cell differentiation. BCR signal strength may also influence B cell fate decisions. Here, we used the Epstein-Barr virus protein LMP2A as a constitutively active BCR surrogate to study the contribution of BCR signal strength in B cell differentiation. Mice carrying a targeted replacement of Igh by LMP2A leading to high or low expression of the LMP2A protein developed B-1 or follicular and marginal zone B cells, respectively. These data indicate that BCR signal strength, rather than antigen specificity, determines mature B cell fate. Furthermore, spontaneous germinal centers developed in gut-associated lymphoid tissue of LMP2A mice, indicating that microbial antigens can promote germinal centers independently of BCR-mediated antigen recognition.

Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1

The transcription factor X-box binding protein 1 (XBP-1) is essential for the differentiation of plasma cells and the unfolded protein response (UPR). Here we show that UPR-induced splicing of XBP-1 by the transmembrane endonuclease IRE1 is required to restore production of immunoglobulin in XBP-1-/- mouse B cells, providing an integral link between XBP-1, the UPR and plasma cell differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP-1, whereas its post-transcriptional processing is dependent on synthesis of immunoglobulins during B cell differentiation. We also show that XBP-1 is involved in controlling the production of interleukin-6, a cytokine that is essential for plasma cell survival. Thus, signals upstream and downstream of XBP-1 integrate plasma cell differentiation with the UPR.

CD22 regulates B cell receptor-mediated signals via two domains that independently recruit Grb2 and SHP-1

Recognition of antigen by the B cell antigen receptor (BCR) determines the subsequent fate of a B cell and is regulated in part by the involvement of other surface molecules, termed coreceptors. CD22 is a B cell-restricted coreceptor that gets rapidly tyrosyl-phosphorylated and recruits various signaling molecules to the membrane following BCR ligation. Although CD22 contains three immunoreceptor tyrosine-based inhibitory motifs (ITIMs), only the two carboxyl-terminal ITIM tyrosines are required for efficient recruitment of the SHP-1 phosphatase after BCR ligation. Furthermore, Grb2 is inducibly recruited to CD22 in human and murine B cells. Unlike SHP-1, Grb2 recruitment to CD22 is not inhibited by specific doses of the Src family kinase-specific inhibitor PP1. The tyrosine residue in CD22 required for Grb2 recruitment (Tyr-828) is distinct and independent from the two ITIM tyrosines required for efficient SHP-1 recruitment (Tyr-843 and Tyr-863). Individually both Lyn and Syk are required for maximal phosphorylation of CD22 following ligation of the BCR, and together Lyn and Syk are required for all of the constitutive and induced tyrosine phosphorylation of CD22. We propose that the cytoplasmic tail of CD22 contains two domains that regulate signal transduction pathways initiated by the BCR and B cell fate.

B cells with the guts to switch

Expression of mIgM was thought to be esential for the differentiation of B cells expressing antibodies of other classes. New evidence suggests isotype class switching to IgA can occur in the absence of mIgM.

Polygenic autoimmune traits: Lyn, CD22, and SHP-1 are limiting elements of a biochemical pathway regulating BCR signaling and selection

A B lymphocyte hyperactivity syndrome resembling systemic lupus erythematosus characterizes mice lacking the src-family kinase Lyn. Lyn is not required to initiate B cell antigen receptor (BCR) signaling but is an essential inhibitory component. lyn-/- B cells have a delayed but increased calcium flux and exaggerated negative selection responses in the presence of antigen and spontaneous hyperactivity in the absence of antigen. As in invertebrates, genetic effects of loci with only one functional allele can be used to analyze signaling networks in mice, demonstrating that negative regulation of the BCR is a complex quantitative trait in which Lyn, the coreceptor CD22, and the tyrosine phosphatase SHP-1 are each limiting elements. The biochemical basis of this complex trait involves a pathway requiring Lyn to phosphorylate CD22 and recruit SHP-1 to the CD22/BCR complex.

CD22 regulates thymus-independent responses and the lifespan of B cells

The B-lymphocyte-restricted glycoprotein CD22 is expressed on mature IgM+IgD+ B cells, and is capable of binding to ligands on T and B cells. CD22 can interact with both the B-cell antigen receptor (BCR) complex and signalling molecules, including the protein tyrosine phosphatase SHP1 (PTP1C, SHP), a putative negative regulator of BCR signalling. Thus CD22 may facilitate interactions with lymphocytes and regulate the threshold of BCR signalling. To define the in vivo function of CD22, we generated CD22-deficient mice. Here we show that CD22 is required for normal antibody responses to thymus-independent antigens and regulates the lifespan of mature B cells.