IL-2 enhances effector function but suppresses follicular localization of CD8⁺ T cells in chronic infection

Reviewer #1 (Public Review):

Summary:

The title states "IL-2 enhances effector function but suppresses follicular localization of CD8+ T cells in chronic infection" which data from the paper show but does not seem to be the major goal of the authors. As stated in the short assessment above, the goal of this work seems to connect IL-2 signals, mostly given exogenously, to the differentiation of progenitor T cells (TPEX) that will help sustain effector T cell responses against chronic viral infection (TEX/TEFF). The authors mostly use chronic LCMV infection in mice as their model of choice, Flow cytometry, fluorescent microscopy, and some in vitro assays to explore how IL2 regulates TPEX and TEX/TEFF differentiation. Gain and loss of functions experiments are also conducted to explore the roles of L2 signaling and BLIMP-1 in regulating these processes. Lastly, a loose connection of their mouse findings on TPEX/TEX cells to a clinical study using low-dose IL-2 treatment in SLE patients is attempted.

Strengths:

(1) The impact of IL-2 treatment of TPEX/TEX differentiation is very clear.

(2) The flow cytometry data are convincing and state-of-the-art.

Weaknesses:

(1) The title appears disconnected from the major focus of the work.

(2) The number of TPEX cells is not changed. IL2 treatment increases the number of TEFF and the proportion of TPEX is lower suggesting it does not target TPEX formation. The conclusion about an inhibitory role of IL2 treatment on TPEX formation seems therefore largely overstated.

(3) Are the expanded TEX/TEFF cells really effectors? Only GrB and some cell surface markers are monitored (44, 62L). Other functions should be included, e.g., CD107a, IFNg, TNF, chemokines - Tbet?

(4) The rationale for IL2 treatment timing is unclear. Seems that this is given at the T cell contraction time and this is interesting compared to the early treatment that ablate TPEX generation. Maybe this should really be explored further?

(5) The TGFb/IL6/IL2 in vitro experiment does not bring much to the paper.

(6) The Figure 2 data try to provide an explanation for a prior lack of difference in viral titers after IL2 treatment. It is hard to be convinced by these tissue section data as presented. It also begs the question of how the host would benefit from the low dose IL-2 treatment if IL-2 TEFF are not contributing to viral control as a result of their inappropriate localization to viral reservoirs.

(7) It is unclear what the STA5CA and BLIMP-1 KO experiments in Figure 3 add to the story that is not already expected/known.

(8) The connection to the low-dose IL2 treatment in SLE patients is very loose and weak. This version is likely not the ligand that preferentially signals to CD122 either. SLE is different from a chronic viral infection and the question of timing seems critical from all the data shown in this manuscript. So it is very difficult to make any robust link to the mechanistic data.

(9) It is really unclear what the take-home message is. IL-2 is signaling via STAT5 and BLIMP1 is also a known target as published by many groups including this one, and these results are more than expected. The observation that TEFF may be differentially localized in the WP area is interesting but no mechanisms are really provided (guessing CXCR5 but again expected). Also, all these observations are highly dependent on the timing of IL2 administration which is fascinating but not explored at all. It also limits significance since underlying mechanisms are unknown and we do not know when such treatment would have to be given.

Reviewer #2 (Public Review):

This study utilized the LCMV Docile infection model, which induces chronic and persistent infection in mice, leading to T cell exhaustion and dysfunction. Through exogenous IL-2 fusion protein treatment during the late stage of infection, the researchers found that IL-2 treatment significantly enlarges the antigen-specific effector CD8 T cells, expanding the CXCR5-TCF1- exhausted population (Tex) while maintaining the size of the CXCR5+TCF1+ precursors of exhausted T cell population (Tpex). This preservation of the Tpex population's self-renewing capacity allows for sustained T cell proliferation and antiviral responses.

The authors discovered a dual effect of IL-2 treatment: it decreases CXCR5 expression on Tpex cells, restricting their entry into the B cell follicle. This may explain why IL-2 treatment has little impact on overall viral control. However, this finding also suggests a potential application of IL-2 treatment for autoimmune diseases, as it can suppress specific immune responses within the B cell follicle. Using imaging-based approaches, the team provided direct evidence that IL-2 treatment shifts the viral load to concentrate within the B cell follicle, correlating with the observed decrease in CXCR5 expression.

Further, the researchers showed that ectopic expression of constitutively active STAT5, downstream of IL-2 induced cytokine signaling, in P14 TCR transgenic T cells (specific for an LCMV epitope), drove the T cell population toward the CXCR5- Tex phenotype over the CXCR5+ Tpex cells in vivo. Additionally, abrogating Blimp1, upregulated by active IL-2-phosphorylated STAT5 signaling, restored the CXCR5+ Tpex population.

Building on these results, the researchers used an engineered IL-2 fusion protein, ANV410, targeting the beta-chain of the IL-2 receptor CD122, which successfully replicated their earlier findings. Importantly, the Tpex-sustaining effect of IL-2 was only observed when treatment was administered during the late stage of infection, as early treatment suppressed Tpex cell generation. Immune profiling of SLE patients undergoing low-dose IL-2 treatment showed a similar reduction in the CXCR5+ Tpex cell population.

This study provides compelling data on the physiological consequences of IL-2 treatment during chronic viral infection. By leveraging the chronic and persistent LCMV Docile infection model, the researchers identified the temporal effects of IL-2 fusion protein treatment, offering strategic insights for therapies targeting cancer and autoimmune diseases.