Playing chess with human cytomegalovirus

Human cytomegalovirus (HCMV) is a common infection worldwide which persists in infected individuals despite a massive host immune response which is partially protective against a high viral load and serious disease. Antibodies are directed against surface glycoproteins, cell‐mediated immunity (CMI) is directed towards peptides derived from structural and non‐structural proteins, but the targets of antibody‐dependent cellular cytotoxicity (ADCC) have not been well defined. The ability of this virus to persist in the face of a massive immune response is explained by its propensity to spread cell to cell, to avoid an extracellular environment rendered inhospitable by neutralising antibodies, while expressing multiple immune evasion genes to limit the effect of CMI. Laboratory studies usually rely on scientific reductionism to isolate individual components and define their mechanism of action. In this way, gene products (micro‐ RNAs as well as proteins) encoded within the HCMV genome have been shown to inhibit NK cells, macrophages and antibody‐ mediated neutralisation, while others send decoy signals to NK cells and macrophages to prevent them attacking virus‐infected cells. Collectively, they are termed immune evasins, and multiple elegant studies demonstrate how individual gene products achieve these objectives. Some of them are called UL‐binding proteins because the product of the 16th gene in the unique long (UL) region of the genome was the first to be defined. Remarkably, approximately 70% of the large genome of HCMV is committed to immune evasion. The diverse components of the immune system do not work in isolation; in the intact human, they may interact and cooperate to partially control HCMV replication, but we have little detailed knowledge about this. In a recent paper, authors with a strong background in examining many of these functions individually, assembled together some of the immune functions that require collaborative contributions from more than one component of the immune system. Specifically, they studied viral proteins expressed at the surface of the infected cell and determined when antibody plus NK cells could mediate ADCC. Remarkably, these targets were not the major structural glycoproteins of the virus, but the proteins it deploys as immune evasins. Leads were followed up by making human monoclonal antibodies (mAbs) against identified surface proteins and their potency confirmed. The temporal expression of viral proteins was employed to identify potential candidate target proteins, especially those produced early in the replicative cycle before virion production. Each of 15 candidates was cloned into a replication‐deficient adenovirus and tested for ADCC in the presence of a commercial source of human IgG from individuals with past HCMV infection. For the next stage, they prioritised two of these proteins (from genes UL16 and UL141), which were both type I membrane proteins, and cloned them into separate adenoviruses. Purified proteins were extracted, labelled and used to determine the abundance of B cells that recognised these proteins. The cells were sorted and propagated to generate plasma cells. After deletion of an endoplasmic reticulum retention signal, 60 B cell supernatants bound UL141 (nine examples) or UL16 (five examples). The immunoglobulin genes were cloned into an IgG1 background and expressed. Interestingly, these mAbs were more potent in ADCC assays against the cognate antigen expressed in fibroblasts than by flow cytometry. The mAbs were unable to directly activate NK cells using targets infected with HCMV. Careful experiments showed this was not due to the Fc receptors encoded by HCMV. The introduction of two amino acid changes into the cloned monoclonal antibodies (previously shown to enhance binding to CD16, a major cellular Fc receptor) produced highly potent mAbs. Experiments with cells radiolabelled with Cr extended the observations from activation of NK cells to lysis of targets. Candidates were thus identified with the potential to proceed to development for clinical applications. Overall, the findings of this tour de force paper provide insight into the evolution of HCMV that could be described as serial moves in a game of chess. The host starts by attacking the initial virus infection using conventional humoral immunity and CMI, so HCMV develops immune evasins as a way of avoiding these responses. The host then mounts a response against the immune evasins using a different aspect of the immune repertoire, ADCC. The host needs to coordinate all of its weapons to keep HCMV suppressed to low levels, while the virus needs all of its countermeasures to survive sufficiently to transmit to the next host. The game settles into a stalemate where neither can gain the upper hand, but neither has lost the game. The virus can spread slowly around the world, while the host can be satisfied that the human race can persist despite the ubiquitous presence of this virus. Like political leaders of superpowers, the two protagonists can strut their stuff in typical alpha male fashion by each claiming that they have won the war. Reports of occasional cases of serious disease in individuals with impaired or immature immune systems