Apoptosis and necroptosis are complementary pathways controlled by common signaling adaptors, kinases and proteases; among these, caspase-8 (Casp8) is critical for death receptor (DR)-induced apoptosis. This caspase has also been implicated in nonapoptotic pathways that regulate Fas-associated via death domain (FADD)-dependent signaling and other less defined biological processes as diverse as innate immune signaling and myeloid or lymphoid differentiation patterns 1. Casp8 suppresses RIP3/RIP1 kinase complex-dependent 2–4 necroptosis 5 that follows DR-activation as well as a RIP3-dependent, RIP1-independent necrotic pathway that has emerged as a host defense mechanism against murine cytomegalovirus (MCMV) 6. Disruption of Casp8 expression leads to embryonic lethality in mice between E10.5 and E11.5 7. Thus, Casp8 may naturally hold alternative RIP3-dependent death pathways in check in addition to its role promoting apoptosis. We find that RIP3 is responsible for the midgestational death of Casp8-deficient embryos. Remarkably, Casp8−/−Rip3−/− double mutant mice are viable and mature into fertile adults with a full immune complement of myeloid and lymphoid cell types. These mice appear immunocompetent but develop lymphadenopathy by four months of age marked by accumulation of abnormal T cells in the periphery, a phenotype reminiscent of mice with Fas-deficiency (lpr/lpr). Casp8 contributes to homeostatic control in the adult immune system; however, RIP3 and Casp8 are together completely dispensable for mammalian development.

The innate antiviral factor TRIM5␣ restricts the replication of some retroviruses through its interaction with the viral capsid protein, leading to abortive infection. While overexpression of human TRIM5␣ results in modest restriction of human immunodeficiency virus type 1 (HIV-1), this inhibition is insufficient to block productive infection of human cells. We hypothesized that polymorphisms within TRIM5 may result in increased restriction of HIV-1 infection. We sequenced the TRIM5 gene (excluding exon 5) and the 4.8-kb 5 putative regulatory region in genomic DNA from 110 HIV-1-infected subjects and 96 exposed seronegative persons, along with targeted gene sequencing in a further 30 HIV-1-infected individuals. Forty-eight single nucleotide polymorphisms (SNPs), including 20 with allele frequencies of >1.0%, were identified. Among these were two synonymous and eight nonsynonymous coding polymorphisms. We observed no association between TRIM5 polymorphism in HIV-1-infected subjects and their set-point viral load after acute infection, although one TRIM5 haplotype was weakly associated with more rapid CD4 ؉ T-cell loss. Importantly, a TRIM5 haplotype containing the nonsynonymous SNP R136Q showed increased frequency among HIV-1-infected subjects relative to exposed seronegative persons, with an odds ratio of 5.49 (95% confidence interval ‫؍‬ 1.83 to 16.45; P ‫؍‬ 0.002). Nonetheless, we observed no effect of individual TRIM5␣ nonsynonymous mutations on the in vitro HIV-1 susceptibility of CD4 ؉ T cells. Therefore, any effect of TRIM5␣ polymorphism on HIV-1 infection in primary lymphocytes may depend on combinations of SNPs or on DNA sequences in linkage disequilibrium with the TRIM5␣ coding sequence.

Summary Programmed necrosis mediated by receptor interacting protein kinase (RIP)3 (also called RIPK3) has emerged as an alternate death pathway triggered by TNF family death receptors, pathogen sensors, interferon receptors, Ag-specific TCR activation and genotoxic stress. Necrosis leads to cell leakage and acts as a ‘trap door’, eliminating cells that cannot die by apoptosis due to the elaboration of pathogen-encoded caspase inhibitors. Necrotic signaling requires RIP3 binding to one of three partners, RIP1, DAI or TRIF, via a common RIP homotypic interaction motif (RHIM). Once activated, RIP3 kinase targets the pseudokinase MLKL to drive cell lysis. Although necrotic or apoptotic death can enhance T cell cross-priming during infection, mice that lack these extrinsic programmed cell death pathways are able to produce antigen-specific T cells and control viral infection. The entwined relationship of apoptosis and necrosis evolved in response to pathogen-encoded suppressors to support host defense and contribute to inflammation.

Exposed seronegative individuals (ES) with persistent high-risk sexual behavior may be less susceptible to human immunodeficiency virus type 1 (HIV-1) infection because they carry the chemokine receptor (CR) gene alleles CCR5 open reading frame (ORF) ⌬32, CCR5 promoter ؊2459G, or CCR2 ORF 64I (CCR2-64I), all of which have been found to diminish HIV-1 infectivity and/or disease progression. To investigate this, we determined the haplotypes for these three genetic loci in 93 ES and 247 low-risk control individuals. To test if protective haplotypes exert their effect by modulating CR expression, we measured the protein expression of CCR5 and CXCR4 on circulating CD4؉ T cells and CD14 ؉ monocytes in 71 ES and 92 controls. To avoid investigator bias, the analysis was performed without knowledge of each subject's risk and genotype. The CCR5 ؊2459G allele was significantly enriched in ES Caucasian men, who constituted the majority (84%) of the ES cohort, compared to the control Caucasian men (P ‫؍‬ 0.02). This increase was mostly attributable to a higher frequency of the ؊2459 A/G versus the ؊2459 A/A genotype in individuals heterozygous for the ⌬32 allele (P ‫؍‬ 0.012). No protective influence of the CCR2-64I allele was observed. The haplotypes CCR5 ORF ⌬32/CCR5 ؊2459A (in complete linkage disequilibrium) and CCR5 ORF wt/CCR5 ؊2459G had a cumulative negative effect on the expression of CCR5, since we measured significantly reduced CCR5 densities on both T-helper cells and monocytes only when both haplotypes were present. Densities of CCR5 on lymphocytes and monocytes were correlated (r ‫؍‬ 0.59; P < 0.0001), indicating concordance of CCR5 expression patterns across different cell types. We conclude that the CCR5 ORF ⌬32/wt-CCR5 ؊2459 A/G genotype combination offers an advantage in resisting sexual HIV-1 transmission and that this effect is mediated by a relative paucity of CCR5 on potential target cells of HIV-1.