Double‐Edged Genetic Swords and Immunity: Lesson from CCR5 and Beyond. Part 2
Murphy’s group showed that after challenge with WNV, CCR5‐null mice had markedly increased viral titers in the central nervous system and had increased mortality compared with that of wild‐type mice, thus suggesting that CCR5 expression was necessary to mount a strong host defense against WNV. Subsequently, they demonstrated that there was a strong epidemiologic association between homozygosity for CCR532 and WNV in humans.
However, because they were unable to distinguish in their previous studies whether the observations were associated with susceptibility to acquiring WNV or associated with the severity of clinical presentation, they conducted the present study. Lim et al now show that the prevalence of CCR5Δ32/Δ32 was comparable in case patients with WNV infection and control participants, which suggests that this genotype is not a susceptibility factor for acquiring WNV infection. However, among the case patients, those patients who were homozygous for CCR532 experienced significantly more symptoms, on average, than did those patients who were heterozygous for CCR532 or who had wild‐type CCR5 genotype. These data indicate that the CCR5 null state is a risk factor for more pronounced early clinical manifestations after infection with WNV.
A noteworthy aspect of the present report by Lim et al is the study design. The case patients and control participants were derived from 35 million blood donors who were screened for WNV. This contrasts with their prior studies, which examined subjects who sought medical attention for symptomatic disease and were compared with otherwise healthy subjects. Also minimizing selection bias, both case patients and control participants in this report were administered the same standardized symptom questionnaire before disclosure of their true WNV infection status, and this study feature facilitated evaluation of the association of the CCR5 null state with the number and severity of early symptoms of WNV infection.
What will Act III reveal? Readers are referred to some possibilities posited in recent opinion pieces. We focus on 4 points. First, the present study raises a pathogenic conundrum: why does the CCR5 null state confer risk for a more aggressive disease but not associate with risk of acquiring WNV infection (ie, viral entry)? One possibility is that CCR5‐mediated signaling events generate critical immune responses that contain the spread of infection but are irrelevant for the initial entry of WNV. In this regard, there are abundant in vitro data linking CCR5 and its ligands to T cell immunity, and 2 recent studies provide corroborative in vivo data: first, that both humans and mice lacking CCR5 surface expression display reduced delayed‐type hypersensitivity skin test responses (an in vivo correlate of T cell function and interleukin 2 [IL‐2] production), and second, that CCR5 expression regulates T cell proliferation, as well as IL‐2 and CD25 expression during T lymphocyte activation. Notably, T cells from CCR5‐null mice secrete lower amounts of IL‐2 than do wild‐type mice; a similar phenotype is observed in CCR5Δ32 homozygotes, as well as after Ab‐mediated blockade of CCR5 in human T cells genetically intact for CCR5 expression. These studies underscore that CCR5 expression may influence clinical outcomes after viral infection by affecting parameters (eg, T cell immunity) that are independent of viral entry. This may have relevance to antiviral immune responses to flaviviruses, including WNV, because CD4+ T cells have a critical function in the control and resolution of primary WNV infection; a strong Th1 T cell response, as characterized by interferon and IL‐2 production, results in reduction of neurological sequelae. Thus, one possibility is that the phenotype of “low CCR5 expression–low IL‐2 levels” may contribute to WNV pathogenesis. Hence, we anticipate that Act III will define the precise mechanisms by which CCR5 influences antiviral responses to flaviviruses as well as to lentiviruses.