Low-Dose Interleukin-2 Therapy Activates Circulating T Follicular Regulatory Cells and Suppresses Circulating T Follicular Helper Cells in Patients with Chronic Gvhd

Introduction: T follicular helper (TFH) and T follicular regulatory (TFR) cells play important roles in the regulation of B-cell immunity. While TFH promote B cell functions in the germinal center (GC), TFR function as negative regulators of the GC response. Previous studies in murine models established that TFH and GC B cells are required for the development of chronic graft-versus-host disease (cGVHD). We previously reported that circulating TFH (cTFH) were more functionally activated in patients with active cGVHD compared with patients with no cGVHD. Low-dose IL-2 therapy has been shown to selectively expand CD4Treg and improve cGVHD symptoms. In the current study, we examined the effects of IL-2 therapy on cTFH and circulating TFR (cTFR) in patients with steroid resistant cGVHD.

Methods: Single cell mass cytomtery (CyTOF) was performed on cryopreserved peripheral blood mononuclear cells (PBMC) from healthy donors and 17 adult patients with active cGVHD receiving daily low-dose IL-2 therapy (Koreth et al. Blood 2016). A panel of 35 metal-tagged monoclonal antibodies was used to simultaneously examine the phenotypic and functional effects of low-dose IL-2 on lymphocyte populations in vitro and in vivo. The analytic panel included 22 cell surface markers to identify distinct lymphocyte subsets and 13 intracellular markers to measure functional status and activation of specific signaling pathways. Before staining for surface and intracellular antigens, serial samples from individual patients were barcoded to ensure uniformity of analysis. viSNE was used to visualize of high-dimensional data on a two-dimensional map and quantify single cell mass cytometry data.

Results: In PBMC from healthy donors, expression of CD25 (IL-2Rα), CD95, CTLA-4, BLIMP-1 and GITR was higher in cTFR compared with cTFH. To examine the response to IL-2 in vitro, PBMC from healthy donors were stimulated with IL-2 for 15 minutes (Figure 1A). At low IL-2 concentrations (1 to 10 IU/mL), phospho-STAT5 (p-STAT5) was selectively activated in cTFR compared with cTFH. At high IL-2 concentrations (100 to 1,000 IU/mL), p-STAT5 was activated in both cTFR and cTFH. To examine the response to IL-2 in vivo, we used mass cytometry to examine serial PBMC samples from cGVHD patients receiving daily low dose IL-2 therapy (1x10⁶ IU/M²/day). Selective expansion of cTFR was noted after 1 week of treatment and cTFR expansion remained stable for the 12 week duration of therapy. Expanded cTFR increased expression of p-STAT5, FoxP3, BCL6, HLA-DR (Figure 1B) and CD25, CD95, CTLA-4, ICOS, Ki67 and Helios 1 week after starting IL-2. cTFR:cTFH ratio increased rapidly after starting low dose IL-2 and paralleled the increased Treg:Tcon ratio (Figure 1C). Activated TFH and TFR can be identified by expression of ICOS and PD-1. The expansion of ICOS+PD-1+ cTFR was evident after 1 week of IL-2 and remained elevated at the end of therapy. In contrast, ICOS+PD-1+ cTFH increased 1 week after starting IL-2 therapy but subsequently decreased and fell below baseline 6 and 12 weeks after starting IL-2 (Figure 1D). Activated ICOS+PD-1+ cTFR expressed higher levels of p-STAT5, BCL-6, FoxP3, HLA-DR and CD25 during low dose IL-2 therapy. In contrast, these functional markers were not increased in ICOS+PD-1+ cTFH during IL-2 therapy (Figure 1B).

Conclusion: Single cell mass cytometry analysis revealed that daily low dose IL-2 therapy induces selective activation and increased expression of functional proteins in ICOS+PD-1+ cTFR. In contrast, activated ICOS+PD-1+ cTFH were suppressed during IL-2 therapy. The selective activation of cTFR and suppression of cTFH provide a mechanism whereby low dose IL-2 therapy can promote B cell tolerance as well as T cell tolerance in patients with cGVHD.

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