Responding to stimuli, nucleotide-binding domain and leucine-rich repeat-containing proteins (NLRs) oligomerize into multiprotein complexes, termed inflammasomes, mediating innate immunity. Recognition of bacterial pathogens by NLR apoptosis inhibitory proteins (NAIPs) induces NLR family CARD domain-containing protein 4 (NLRC4) activation and formation of NAIP-NLRC4 inflammasomes. The wheel-like structure of a PrgJ-NAIP2-NLRC4 complex determined by cryogenic electron microscopy at 6.6 angstrom reveals that NLRC4 activation involves substantial structural reorganization that creates one oligomerization surface (catalytic surface). Once activated, NLRC4 uses this surface to catalyze the activation of an inactive NLRC4, self-propagating its active conformation to form the wheel-like architecture. NAIP proteins possess a catalytic surface matching the other oligomerization surface (receptor surface) of NLRC4 but not those of their own, ensuring that one NAIP is sufficient to initiate NLRC4 oligomerization.

Antigen-presenting cells (APCs) induce T cell activation as well as T cell tolerance. The molecular basis of the regulation of this critical ‘decision’ is not well understood. Here we show that HDAC11, a member of the HDAC histone deacetylase family with no prior defined physiological function, negatively regulated expression of the gene encoding interleukin 10 (IL-10) in APCs. Overexpression of HDAC11 inhibited IL-10 expression and induced inflammatory APCs that were able to prime naive T cells and restore the responsiveness of tolerant CD4+ T cells. Conversely, disruption of HDAC11 in APCs led to upregulation of expression of the gene encoding IL-10 and impairment of antigen-specific T cell responses. Thus, HDAC11 represents a molecular target that influences immune activation versus immune tolerance, a critical ‘decision’ with substantial implications in autoimmunity, transplantation and cancer immunotherapy.

Single gene mutations in β integrins can account for functional defects of individual cells of the hematopoietic system. In humans, mutations in β 2 integrin lead to leukocyte adhesion deficiency (LAD) syndrome and mutations in β 3 integrin cause the bleeding disorder Glanzmann thrombasthenia. However, multiple defects in blood cells involving various β integrins (β 1 , β 2 , and β 3 ) occur simultaneously in patients with the recently described LAD type III (LAD-III). Here we show that the product of a single gene, Ca 2+ and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI), controlled the activation of all 3 integrins in the hematopoietic system. Neutrophils from CalDAG-GEFI -/-mice exhibited strong defects in Rap1 and β 1 and β 2 integrin activation while maintaining normal calcium flux, degranulation, and ROS generation. Neutrophils from CalDAG-GEFIdeficient mice failed to adhere firmly to stimulated venules and to migrate into sites of inflammation. Furthermore, CalDAG-GEFI regulated the activation of β 1 and β 3 integrins in platelets, and CalDAG-GEFI deficiency caused complete inhibition of arterial thrombus formation in mice. Thus, mice engineered to lack CalDAG-GEFI have a combination of defects in leukocyte and platelet functions similar to that of LAD-III patients.

The central step in eukaryotic homologous recombination (HR) is ATP-dependent DNA strand exchange mediated by the Rad51 recombinase. In this process, Rad51 assembles on single-stranded DNA (ssDNA) to yield a helical filament that is able to search for and invade a homologous double-stranded DNA (dsDNA), leading to strand separation in the dsDNA and formation of new base pairs between the initiating ssDNA and the complementary strand in the duplex partner. Here we have used cryo-electron microscopy (cryo-EM) to solve the structures of human RAD51 in complex with DNA molecules in their presynaptic and post-synaptic states at near atomic resolution. Our structures revealed both conserved and distinct structural features of the RAD51-DNA complexes in comparison with its prokaryotic counterpart. Importantly, we have also captured the structure of an arrested synaptic complex. The results provide insights into the molecular mechanism of DNA homology search and strand exchange processes.