Mechanistic impact of oligomer poisoning by dominant-negative CARD11 variants.

The CARD11 scaffold controls antigen receptor signaling to NF-κB, JNK, and mTOR. Three classes of germline mutations in CARD11 cause Primary Immunodeficiency, including homozygous loss-of-function (LOF) mutations in CARD11 deficiency, heterozygous gain-of-function (GOF) mutations in BENTA disease, and heterozygous dominant-negative LOF mutations in CADINS. Here, we characterize LOF CARD11 mutants with a range of dominant-negative activities to identify the mechanistic properties that cause these variants to exert dominant effects when heterozygous. We find that strong dominant negatives can poison signaling from mixed wild-type:mutant oligomers at two steps in the CARD11 signaling cycle, at the Opening Step and at the Cofactor Association Step. Our findings provide evidence that CARD11 oligomer subunits cooperate in at least two steps during antigen receptor signaling and reveal how different LOF mutations in the same oligomeric signaling hub may cause disease with different inheritance patterns.

Pathway-Specific Defects in T, B, and NK Cells and Age-Dependent Development of High IgE in Mice Heterozygous for a CADINS-Associated Dominant Negative CARD11 Allele.

CARD11 is a multidomain scaffold protein required for normal activation of NF-κB, JNK, and mTOR during Ag receptor signaling. Germline CARD11 mutations cause at least three types of primary immunodeficiency including CARD11 deficiency, B cell expansion with NF-κB and T cell anergy (BENTA), and CARD11-associated atopy with dominant interference of NF-κB signaling (CADINS). CADINS is uniquely caused by heterozygous loss-of-function CARD11 alleles that act as dominant negatives. CADINS patients present with frequent respiratory and skin infections, asthma, allergies, and atopic dermatitis. However, precisely how a heterozygous dominant negative CARD11 allele leads to the development of this CADINS-specific cluster of symptoms remains poorly understood. To address this, we generated mice expressing the CARD11 R30W allele originally identified in patients. We find that CARD11R30W/+ mice exhibit impaired signaling downstream of CARD11 that leads to defects in T, B, and NK cell function and immunodeficiency. CARD11R30W/+ mice develop elevated serum IgE levels with 50% penetrance that becomes more pronounced with age, but do not develop spontaneous atopic dermatitis. CARD11R30W/+ mice display reduced regulatory T cell numbers, but not the Th2 expansion observed in other mice with diminished CARD11 activity. Interestingly, the presence of mixed CARD11 oligomers in CARD11R30W/+ mice causes more severe signaling defects in T cells than in B cells, and specifically impacts IFN-γ production by NK cells, but not NK cell cytotoxicity. Our findings help explain the high susceptibility of CADINS patients to infection and suggest that the development of high serum IgE is not sufficient to induce overt atopic symptoms.

Coordinated regulation of scaffold opening and enzymatic activity during CARD11 signaling.

The activation of key signaling pathways downstream of antigen receptor engagement is critically required for normal lymphocyte activation during the adaptive immune response. CARD11 is a multidomain signaling scaffold protein required for antigen receptor signaling to NF-κB, c-Jun N-terminal kinase, and mTOR. Germline mutations in the CARD11 gene result in at least four types of primary immunodeficiency, and somatic CARD11 gain-of-function mutations drive constitutive NF-κB activity in diffuse large B cell lymphoma and other lymphoid cancers. In response to antigen receptor triggering, CARD11 transitions from a closed, inactive state to an open, active scaffold that recruits multiple signaling partners into a complex to relay downstream signaling. However, how this signal-induced CARD11 conversion occurs remains poorly understood. Here we investigate the role of Inducible Element 1 (IE1), a short regulatory element in the CARD11 Inhibitory Domain, in the CARD11 signaling cycle. We find that IE1 controls the signal-dependent Opening Step that makes CARD11 accessible to the binding of cofactors, including Bcl10, MALT1, and the HOIP catalytic subunit of the linear ubiquitin chain assembly complex. Surprisingly, we find that IE1 is also required at an independent step for the maximal activation of HOIP and MALT1 enzymatic activity after cofactor recruitment to CARD11. This role of IE1 reveals that there is an Enzymatic Activation Step in the CARD11 signaling cycle that is distinct from the Cofactor Association Step. Our results indicate that CARD11 has evolved to actively coordinate scaffold opening and the induction of enzymatic activity among recruited cofactors during antigen receptor signaling.

Mechanisms of Regulated and Dysregulated CARD11 Signaling in Adaptive Immunity and Disease.

CARD11 functions as a key signaling scaffold that controls antigen-induced lymphocyte activation during the adaptive immune response. Somatic mutations in CARD11 are frequently found in Non-Hodgkin lymphoma, and at least three classes of germline CARD11 mutations have been described as the basis for primary immunodeficiency. In this review, we summarize our current understanding of how CARD11 signals, how its activity is regulated, and how mutations bypass normal regulation to cause disease.