IL-21 is required to control chronic viral infection

Abstract

CD4+ and CD8+ T cell functions are rapidly aborted during chronic infection, preventing viral clearance. CD4+ T cell help is required throughout chronic infection so as to sustain CD8+ T cell responses; however, the necessary factor(s) provided by CD4+ T cells are currently unknown. Using a mouse model of chronic viral infection, we demonstrated that interleukin-21 (IL-21) is an essential component of CD4+ T cell help. In the absence of IL-21 signaling, despite elevated CD4+ T cell responses, CD8+ T cell responses are severely impaired. CD8+ T cells directly require IL-21 to avoid deletion, maintain immunity, and resolve persistent infection. Thus, IL-21 specifically sustains CD8+ T cell effector activity and provides a mechanism of CD4+ T cell help during chronic viral infection.

Fig. 1

IL-21 mRNA expression during viral infection. IL-21 mRNA expression was quantified by real-time reverse transcription polymerase chain reaction after LCMV-Arm or LCMV–Cl 13 infection in (A) uninfected (gray bar), LCMV–Cl 13 infected (black bar), or LCMV–Cl 13 infected and CD4⁺ T cell–depleted (striped bar) splenocytes or in (B) the indicated sorted cell population. Red Bars indicate cells sorted from LCMV-Arm–infected animals; black bars indicate cells sorted from LCMV–Cl 13–infected animals. (C) Frequency of IFN-γ⁺ SMARTA T cells that simultaneously produced IL-2 or IL-21 on day 9 and day 30 after infection. Error bars represent the average ± SD of 3 to 5 mice per group and 2 to 4 independent experiments. *P < 0.05, **P = 0.4.

Fig. 2

IL-21 sustains virus-specific CD8⁺ T cells during viral persistence. (A) LCMV-GP_33–41 MHC class I tetramer staining on days 9 or 30 after LCMV-Arm or LCMV–Cl 13 infection. Numbers indicate the frequency of tetramer⁺ CD8⁺ T cells in WT and Il21r^−/− (KO) mice. (B) Number of tetramer⁺ CD8⁺ T cells on days 9 and 30 after LCMV-Arm or LCMV–Cl 13 infection of WT and Il21r^−/− mice. (C) Number of WT or Il21r^−/− P14 T cells after infection of Il21r^+/+ mice. (D) Frequency of WT and Il21r^−/− LCMV-GP_33–41 tetramer⁺ CD8⁺ T cells in bone-marrow chimeric mice on days 8 and 28 in the blood and on day 28 in the spleen after LCMV–Cl 13 infection. Numbers in parentheses indicate the total proportion of WT and Il21r^−/− CD8⁺ T cells. Bar graphs represent the average ± SD of 3 to 5 mice per group and represent 2 to 4 independent experiments. *P<0.05.

Fig. 3

IL-21 suppresses CD4⁺ T cell function. (A) LCMV-GP_61–80–specific CD4⁺ T cell responses on days 9 and 30 after LCMV-Arm or LCMV–Cl 13 infection of WT or Il21r^−/− mice. Numbers in each quadrant indicate the frequency of IFN-γ⁺ and IL-2⁺ CD4⁺ T cells and the numbers in parentheses indicate the frequency of IFN-γ and IL-2 double-positive cells. (B) The frequency and number of CD4⁺ T cells that simultaneously produce IL-2 and IFN-γ on days 9 or 30 after LCMV-Arm or LCMV–Cl 13 infection. (C) Frequency of LCMV-GP_61–80 tetramer⁺ CD4⁺ T on day 30 after LCMV-Arm or LCMV–Cl 13 infection. The bar graph represents the number of tetramer⁺ CD4⁺ T cells. Bar graphs summarize the average ± SD of 3 to 4 mice per group and 3 to 4 independent experiments. *P < 0.05.

Fig. 4

IL-21 is required to purge chronic viral infection. (A) Serum viral titers after LCMV-Arm (triangles) or LCMV–Cl 13 (circles) infection of WT (black) or Il21r^−/− (red) mice. Data are expressed as plaque-forming units (PFU) per milliliter of serum. The dashed line indicates the lower limit of detection (200 PFU/ml). *P < 0.05 for LCMV–Cl 13 infection of WT versus Il21r^−/− mice. (B and C) The frequency and number of (B) LCMV-GP_33–41 tetramer⁺ CD8⁺ T cells and (C) IFN-γ⁺ CD4⁺ T cells on day 100 after LCMV–Cl 13 infection of WT and Il21r^−/− mice. Time points and bar graphs represent the average ± SD of 3 to 4 mice per group and 3 independent experiments. *P < 0.05.