Antigen-Specific Naive CD8+ T Cells Produce a Single Pulse of IFN-γ In Vivo within Hours of Infection, but without Antiviral Effect

Abstract

In vitro studies have shown that naïve CD8+ T cells are unable to express most of their effector proteins until after at least one round of cell division has taken place. We have re-assessed this issue in vivo, and find that naïve CD8+ T cells mount antigen-specific responses within hours of infection, before proliferation has commenced. Newly-activated naïve antigen specific CD8+ T cells produce a rapid pulse of IFNγ in vivo and begin to accumulate granzyme B and perforin. Later, in vivo cytolytic activity is detectable, coincident with the initiation of cell division. Despite the rapid development of these functional attributes, no antiviral effect was observed early during infection, even when the cells are present in numbers similar to those of virus-specific memory cells. The evolutionary reason for the pulse of IFNγ synthesis by naïve T cells is uncertain, but the lack of antiviral impact suggests that it may be regulatory.

Distinct activation thresholds of human conventional and innate-like memory T cells

“…It is also noteworthy that a TCR signal is sufficient for MAIT cells to elicit a brief TNF-α and IFN-γ pulse 6 hours after TCR stimulation. This is akin to what has been described in naive CD8 + T cells, which also briefly make IFN-γ early after activation and prior to their first cell division (39). We could not detect significant production of IFN-γ in MAIT cells by intracellular cytokine staining at later time points (Figure 1B) or in MAIT cell culture supernatant by Luminex analysis (Figure 2A) 24 hours after stimulation, further arguing that the production is very brief and limited.…”

“…We could not detect significant production of IFN-γ in MAIT cells by intracellular cytokine staining at later time points (Figure 1B) or in MAIT cell culture supernatant by Luminex analysis (Figure 2A) 24 hours after stimulation, further arguing that the production is very brief and limited. A previous study demonstrated that this brief IFN-γ pulse does not result in antiviral immunity, and thus, the purpose is still unclear (39). An IFN-γ response following TCR stimulation was reported in MAIT cell clones and lines (28, 40), which are useful tools to identify antigens but not a suitable substitute for studying ex vivo activation requirements of primary human T cells.…”

TCR independent suppression of CD8 + T cell cytokine production mediated by IFNγ in vivo

“…This rapid viral control, which occurred in the absence of concurrent IFNγ synthesis by CD8 + memory T cells, suggests that the pulse of IFNγ is not directly antiviral and, instead, may be immunoregulatory. This led us also to consider the possibility that IFNγ, itself, might mediate the active, TCR-independent suppression of CD8 + memory T cell responses, because it is abundantly produced by antigen-specific CD8 + T cells immediately following primary viral infection (Hosking et al , 2014), secondary viral infection (Hosking et al , 2013), and peptide stimulation (this study), and a pulse of IFNγ synthesis invariably preceded the onset of the suppressed state. To test this hypothesis, we administered recombinant IFNγ directly to LCMV-immune mice, then infected the mice with LCMV and assessed in vivo IFNγ production by LCMV-specific CD8 + T cells.…”

“…As previously detailed (Foster et al , 2007; Hosking et al , 2013; Hosking et al , 2014; Liu and Whitton, 2005; Whitmire et al , 2008b), 250 µg of Brefeldin A (Sigma St Louis, MO) was injected i.v. into peptide and/or virus challenged LCMV immune mice.…”

“…More specifically, direct IFNγ signaling promotes optimal expansion of primary CD8 + T cells during viral infection (Whitmire et al , 2007; Whitmire et al , 2005), and the capacity of individual CD8 + T cells to produce IFNγ is directly related to their immunodominance and in vitro effector function (Beuneu et al , 2010; Liu et al , 2004). Furthermore, we have recently shown that in vivo production of IFNγ by naïve antigen-specific CD8 + T cells within hours of a primary viral infection is significantly associated with their active proliferation (Hosking et al , 2014). Interestingly, while IFNγR-deficient mice are unable to control LCMV (Bartholdy et al , 2000; Muller et al , 1994; Whitmire et al , 2007), neutralization of IFNγ starting at 3 or 5 days post infection, a period at which antigen-specific lymphocytes become abundant enough to exert detectable anti-viral function, does not limit viral clearance (Wille et al , 1989), suggesting that (i) IFNγ acts very early during primary viral infection to promote expansion and (ii) that ongoing IFNγ production is not required for viral control.…”