Evolutionary Landscapes of Host-Virus Arms Races.

Vertebrate immune systems suppress viral infection using both innate restriction factors and adaptive immunity. Viruses mutate to escape these defenses, driving hosts to counterevolve to regain fitness. This cycle recurs repeatedly, resulting in an evolutionary arms race whose outcome depends on the pace and likelihood of adaptation by host and viral genes. Although viruses evolve faster than their vertebrate hosts, their proteins are subject to numerous functional constraints that impact the probability of adaptation. These constraints are globally defined by evolutionary landscapes, which describe the fitness and adaptive potential of all possible mutations. We review deep mutational scanning experiments mapping the evolutionary landscapes of both host and viral proteins engaged in arms races. For restriction factors and some broadly neutralizing antibodies, landscapes favor the host, which may help to level the evolutionary playing field against rapidly evolving viruses. We discuss the biophysical underpinnings of these landscapes and their therapeutic implications.

NAIP-NLRC4 Inflammasomes Coordinate Intestinal Epithelial Cell Expulsion with Eicosanoid and IL-18 Release via Activation of Caspase-1 and -8.

Intestinal epithelial cells (IECs) form a critical barrier against pathogen invasion. By generation of mice in which inflammasome expression is restricted to IECs, we describe a coordinated epithelium-intrinsic inflammasome response in vivo. This response was sufficient to protect against Salmonella tissue invasion and involved a previously reported IEC expulsion that was coordinated with lipid mediator and cytokine production and lytic IEC death. Excessive inflammasome activation in IECs was sufficient to result in diarrhea and pathology. Experiments with IEC organoids demonstrated that IEC expulsion did not require other cell types. IEC expulsion was accompanied by a major actin rearrangement in neighboring cells that maintained epithelium integrity but did not absolutely require Caspase-1 or Gasdermin D. Analysis of Casp1-/-Casp8-/- mice revealed a functional Caspase-8 inflammasome in vivo. Thus, a coordinated IEC-intrinsic, Caspase-1 and -8 inflammasome response plays a key role in intestinal immune defense and pathology.

Induced proximity of a TIR signaling domain on a plant-mammalian NLR chimera activates defense in plants.

Plant and animal intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors detect pathogen-derived molecules and activate defense. Plant NLRs can be divided into several classes based upon their N-terminal signaling domains, including TIR (Toll-like, Interleukin-1 receptor, Resistance protein)- and CC (coiled-coil)-NLRs. Upon ligand detection, mammalian NAIP and NLRC4 NLRs oligomerize, forming an inflammasome that induces proximity of its N-terminal signaling domains. Recently, a plant CC-NLR was revealed to form an inflammasome-like hetero-oligomer. To further investigate plant NLR signaling mechanisms, we fused the N-terminal TIR domain of several plant NLRs to the N terminus of NLRC4. Inflammasome-dependent induced proximity of the TIR domain in planta initiated defense signaling. Thus, induced proximity of a plant TIR domain imposed by oligomerization of a mammalian inflammasome is sufficient to activate authentic plant defense. Ligand detection and inflammasome formation is maintained when the known components of the NLRC4 inflammasome is transferred across kingdoms, indicating that NLRC4 complex can robustly function without any additional mammalian proteins. Additionally, we found NADase activity of a plant TIR domain is necessary for plant defense activation, but NADase activity of a mammalian or a bacterial TIR is not sufficient to activate defense in plants.

Molecular basis for specific recognition of bacterial ligands by NAIP/NLRC4 inflammasomes.

NLR (nucleotide-binding domain [NBD]- and leucine-rich repeat [LRR]-containing) proteins mediate innate immune sensing of pathogens in mammals and plants. How NLRs detect their cognate stimuli remains poorly understood. Here, we analyzed ligand recognition by NLR apoptosis inhibitory protein (NAIP) inflammasomes. Mice express multiple highly related NAIP paralogs that recognize distinct bacterial proteins. We analyzed a panel of 43 chimeric NAIPs, allowing us to map the NAIP domain responsible for specific ligand detection. Surprisingly, ligand specificity was mediated not by the LRR domain, but by an internal region encompassing several NBD-associated α-helical domains. Interestingly, we find that the ligand specificity domain has evolved under positive selection in both rodents and primates. We further show that ligand binding is required for the subsequent co-oligomerization of NAIPs with the downstream signaling adaptor NLR family, CARD-containing 4 (NLRC4). These data provide a molecular basis for how NLRs detect ligands and assemble into inflammasomes.

Indels allow antiviral proteins to evolve functional novelty inaccessible by missense mutations.

Antiviral proteins often evolve rapidly at virus-binding interfaces to defend against new viruses. We investigated whether antiviral adaptation via missense mutations might face limits, which insertion or deletion mutations (indels) could overcome. We report one such case of a nearly insurmountable evolutionary challenge: the human anti-retroviral protein TRIM5α requires more than five missense mutations in its specificity-determining v1 loop to restrict a divergent simian immunodeficiency virus (SIV). However, duplicating just one amino acid in v1 enables human TRIM5α to potently restrict SIV in a single evolutionary step. Moreover, natural primate TRIM5α v1 loops have evolved indels that confer novel antiviral specificities. Thus, indels enable antiviral proteins to overcome viral challenges inaccessible by missense mutations, revealing the potential of these often-overlooked mutations in driving protein innovation.

Mutational resilience of antiviral restriction favors primate TRIM5α in host-virus evolutionary arms races.

Host antiviral proteins engage in evolutionary arms races with viruses, in which both sides rapidly evolve at interaction interfaces to gain or evade immune defense. For example, primate TRIM5α uses its rapidly evolving 'v1' loop to bind retroviral capsids, and single mutations in this loop can dramatically improve retroviral restriction. However, it is unknown whether such gains of viral restriction are rare, or if they incur loss of pre-existing function against other viruses. Using deep mutational scanning, we comprehensively measured how single mutations in the TRIM5α v1 loop affect restriction of divergent retroviruses. Unexpectedly, we found that the majority of mutations increase weak antiviral function. Moreover, most random mutations do not disrupt potent viral restriction, even when it is newly acquired via a single adaptive substitution. Our results indicate that TRIM5α's adaptive landscape is remarkably broad and mutationally resilient, maximizing its chances of success in evolutionary arms races with retroviruses.

The evolutionary battle between viruses and the immune system is essentially a high-stakes arms race. The immune system makes antiviral proteins, called restriction factors, which can stop the virus from replicating. In response, viruses evolve to evade the effects of restriction factors. To counter this, restriction factors evolve too, and the cycle continues. The challenge for the immune system is that mammals do not evolve as fast as viruses. How then, in the face of this disadvantage, can the immune system hope to keep pace with viral evolution? One human antiviral protein that seems to have struggled to keep up is TRIM5α. In rhesus macaques, it is very effective at stopping the replication of HIV-1 and related viruses. But in humans, it is not effective at all. But why? Protein evolution happens due to small genetic mutations, but not every mutation makes a protein better. If a protein is resilient, it can tolerate lots of neutral or negative mutations without breaking, until it mutates in a way that makes it better. But, if a protein is fragile, even small changes can render it completely unable to do its job. It is possible that restriction factors, like TRIM5α, are evolutionarily 'fragile', and therefore easy to break. But it is difficult to test whether this is the case, because existing mutations have already passed the test of natural selection. This means that either the mutation is somehow useful for the protein, or that it is not harmful enough to be removed. Tenthorey et al. devised a way to introduce all possible changes to the part of TRIM5α that binds to viruses. This revealed that TRIM5α is not fragile; most random mutations increased, rather than decreased, the protein's ability to prevent viral infection. In fact, it appears it would only take a single mutation to make TRIM5α better at blocking HIV-1 in humans, and there are many possible single mutations that would work. Thus, it would appear that human TRIM5α can easily gain the ability to block HIV-1. The next step was to find out whether these gains in antiviral activity are just as easily lost. To do this, Tenthorey et al. performed the same tests on TRIM5α from rhesus macaques and an HIV-blocking mutant version of human TRIM5α. This showed that the majority of random mutations do not break TRIM5α's virus-blocking ability. Thus, TRIM5α can readily gain antiviral activity and, once gained, does not lose it easily during subsequent mutation. Antiviral proteins like TRIM5α engage in uneven evolutionary battles with fast-evolving viruses. But, although they are resilient and able to evolve, they are not always able to find the right mutations on their own. Experiments like these suggest that it might be possible to give them a helping hand. Identifying mutations that help human TRIM5α to strongly block HIV-1 could pave the way for future gene therapy. This step would demand significant advances in gene therapy efficacy and safety, but it could offer a new way to block virus infection in the future.

NLRC4 inflammasome activation is NLRP3- and phosphorylation-independent during infection and does not protect from melanoma.

The NAIP/NLRC4 inflammasome is a cytosolic sensor of bacteria that activates caspase-1 and initiates potent immune responses. Structural, biochemical, and genetic data demonstrate that NAIP proteins are receptors for bacterial ligands, while NLRC4 is a downstream adaptor that multimerizes with NAIPs to form an inflammasome. NLRC4 has also been proposed to suppress tumor growth, though the underlying mechanism is unknown. Further, NLRC4 is phosphorylated on serine 533, which was suggested to be critical for its function. In the absence of S533 phosphorylation, it was proposed that another inflammasome protein, NLRP3, can induce NLRC4 activation. We generated a new Nlrc4-deficient mouse line and mice with S533D phosphomimetic or S533A nonphosphorylatable NLRC4. Using these models in vivo and in vitro, we fail to observe a requirement for phosphorylation in NLRC4 inflammasome function. Furthermore, we find no role for NLRP3 in NLRC4 function, or for NLRC4 in a model of melanoma. These results clarify our understanding of the mechanism and biological functions of NAIP/NLRC4 activation.

The structural basis of flagellin detection by NAIP5: A strategy to limit pathogen immune evasion.

Robust innate immune detection of rapidly evolving pathogens is critical for host defense. Nucleotide-binding domain leucine-rich repeat (NLR) proteins function as cytosolic innate immune sensors in plants and animals. However, the structural basis for ligand-induced NLR activation has so far remained unknown. NAIP5 (NLR family, apoptosis inhibitory protein 5) binds the bacterial protein flagellin and assembles with NLRC4 to form a multiprotein complex called an inflammasome. Here we report the cryo-electron microscopy structure of the assembled ~1.4-megadalton flagellin-NAIP5-NLRC4 inflammasome, revealing how a ligand activates an NLR. Six distinct NAIP5 domains contact multiple conserved regions of flagellin, prying NAIP5 into an open and active conformation. We show that innate immune recognition of multiple ligand surfaces is a generalizable strategy that limits pathogen evolution and immune escape.

NAIP proteins are required for cytosolic detection of specific bacterial ligands in vivo.

NLRs (nucleotide-binding domain [NBD] leucine-rich repeat [LRR]-containing proteins) exhibit diverse functions in innate and adaptive immunity. NAIPs (NLR family, apoptosis inhibitory proteins) are NLRs that appear to function as cytosolic immunoreceptors for specific bacterial proteins, including flagellin and the inner rod and needle proteins of bacterial type III secretion systems (T3SSs). Despite strong biochemical evidence implicating NAIPs in specific detection of bacterial ligands, genetic evidence has been lacking. Here we report the use of CRISPR/Cas9 to generate Naip1(-/-) and Naip2(-/-) mice, as well as Naip1-6(Δ/Δ) mice lacking all functional Naip genes. By challenging Naip1(-/-) or Naip2(-/-) mice with specific bacterial ligands in vivo, we demonstrate that Naip1 is uniquely required to detect T3SS needle protein and Naip2 is uniquely required to detect T3SS inner rod protein, but neither Naip1 nor Naip2 is required for detection of flagellin. Previously generated Naip5(-/-) mice retain some residual responsiveness to flagellin in vivo, whereas Naip1-6(Δ/Δ) mice fail to respond to cytosolic flagellin, consistent with previous biochemical data implicating NAIP6 in flagellin detection. Our results provide genetic evidence that specific NAIP proteins function to detect specific bacterial proteins in vivo.