IL-1R2 Expression in Tfr Cells Controls Allergic Anaphylaxis by Regulating IgG Versus IgE Responses

Abstract

The antibody response is regulated by follicular T helper (Tfh) and regulatory (Tfr) cells that control the germinal center (GC) reaction [1]. We previously showed that Tfh cells express the IL-1 receptor IL-1R1, while Tfr cells express the IL-1 decoy receptor IL-1R2 and IL-1R1. In contrast, regulatory T cells (Tregs) largely do not express IL-1Rs [2]. The role of IL-1Rs expressed in follicular T cells in the regulation of allergy is poorly understood. We recently generated a mouse line in which IL-1R2 is knocked out in FoxP3⁺ T cells (FoxP3^creIL-1R2^lox mice) [3].

Here, we compared these mice to their FoxP3^cre controls in a model of ovalbumin (OVA) sensitization and anaphylaxis. Compared to control mice, FoxP3^creIL-1R2^lox mice displayed a strong exacerbation of allergic anaphylaxis characterized by increased total IgE levels, increased blood basophil-displayed surface IgE density, and significantly enhanced systemic anaphylaxis in response to OVA challenge (Figure 1A–D, Figure S1A–D). Moreover, in vitro OVA-challenged basophils derived from FoxP3^creIL-1R2^lox mice showed increased degranulation compared to controls, demonstrating an increase in reactive IgE (Figure 1E–G, Figure S1F).

In contrast to this enhanced IgE response, FoxP3^creIL-1R2^lox mice generated significantly reduced OVA-specific IgG responses, including IgG1 and IgG2b subclasses (Figure 1H–J). Blockade of the inhibitory IgG receptor FcγRIIb worsened anaphylaxis in control mice but had no effect in FoxP3^creIL-1R2^lox, suggesting that no IgG-dependent protection occurs (Figure 1 K). In vitro, only serum from control mice but not serum from FoxP3^creIL-1R2^lox enhanced FcγR-dependent binding of IgG-OVA complexes to basophils and inhibited basophil degranulation via FcγRIIb (Figure 1L,M, Figure S1G,H). Thus, allergic anaphylaxis in FoxP3^creIL-1R2^lox mice is driven by an increase in reactive IgE and a reduction in protective IgG.

We next investigated the splenic GC response. FoxP3^creIL-1R2^lox mice displayed overall elevated Tfh cell numbers, including IL-4⁺Tfh cells. However, Tfr cells increased even more, resulting in elevated Tfr:Tfh ratios (Figure 2A–C, Figure S3). Looking at B-cell subsets, we noted an increase in plasmablasts (PB), whereas GC Bcell (GCB) numbers were not increased (Figure 2D). Interestingly, GCB displayed reduced levels of proliferation and apoptosis, suggesting overall reduced activity (Figure 2E,F, Figure S3). In line with this notion, GCB from FoxP3^creIL-1R2^lox mice showed reduced OVA binding (Figure 2G).

Finally, isolated splenocytes were re-stimulated with IL-1β and/or OVA. We observed an IL-1R1-dependent activation and proliferation of IL-1R2-deleted Tfr cells, whereas B-cell proliferation upon re-stimulation did not occur in FoxP3^creIL-1R2^lox splenocytes (Figure 2H–K, Figure S4). These results extend our previous report showing that IL-1R2 deletion in Tfr cells enhances their own activation [3]. In agreement with our findings that Tregs do not express IL-1Rs, those cells did not proliferate in response to IL-1 re-stimulation (Figure S4K) [2].

A recent study showed that deletion of IL-1R2 in Tfr cells enhances cellular GC responses and antibodies upon immunization through increasing IL-1 levels [4]. In line with this study, we observe increased follicular T cell numbers upon OVA immunization. However, those only translated to enhanced total IgE responses whereas OVA-specific IgG responses were supressed. These differences could be explained by the here-observed increases in Tfr:Tfh ratios and the different immunization approach. Future studies will resolve this in further detail.

We propose a model in which deletion of IL-1R2 in Tfr cells enhances their own IL-1R1-dependent expansion, resulting in elevated Tfr:Tfh ratios, which may suppress maturation of GCs favoring OVA-specific IgG responses. In contrast, GC quality is sufficient to raise total and even specific IgE responses, potentially enhanced by increases in IL-4⁺Tfh cells. Thus, the deletion of IL-1R2 in Tfr cells leads to a reduction in IgG:IgE ratios which drives the response toward allergic anaphylaxis (Figure 2L).

Our findings are supported by the fact that the IgE response arises through a distinct pathway from IgG, that can occur independent of GCs. IgE switching and PB formation may occur directly from IgM, or post-GC by switching from IgG precursors [5, 6]. Our findings could fit both scenarios, even though the latter would require that sufficient IgG precursors are initially generated during sensitization. Overall, these questions are difficult to answer as long as the details of the IgE response itself remain unclear.

In summary, we show that the level of IL-1R2 expression in Tfr cells may regulate between IgG versus IgE pathways for the GC response, thus controlling allergic anaphylaxis.

D.K. conceptualized and supervised the study. Methodologies were developed by P.E., A.B., G.F., H.D.L., B.B., S.G.D., and D.K. Experiments were conducted by P.E., A.B., R.V., and G.F. Visualization was handled by P.E. Funding was acquired by P.E. and D.K. The original draft was written by P.E., and edited by B.B., S.G.D., and D.K., before a final review by all authors.

The authors declare no conflicts of interest.