Background Immune checkpoint inhibitor (ICI) therapy has revolutionized cancer treatment, however, it can lead to off-target autoimmunity against healthy tissues, known as immune-related adverse events (IRAEs). These events occur in up to 60% of patients.¹ IRAEs can affect any organ system, manifesting as colitis, thyroiditis, hepatitis, and may lead to life-threatening complications such as pneumonitis and myocarditis.² Previous work by our lab and others showed enrichment of T follicular helper (Tfh) cells, a specialized subset of CD4⁺ T cells seen in spontaneous autoimmune diseases, in patients who developed IRAEs.^3–5 In addition, our work implicated Tfh-derived cytokines in the immunopathogenesis of endocrine IRAEs. However, the mechanisms by which ICI therapy causes Tfh cell expansion and activity are poorly understood. By understanding the drivers of Tfh cells during ICI therapy, we can identify targets to modulate their role in ICI autoimmune and anti-tumor immune responses.

Methods Healthy donor peripheral blood mononuclear cells or naïve CD4⁺ T cells were cultured under Tfh-skewing conditions (Activin A, IL-12, anti-CD3/CD28 stimulation) as previously reported,⁶ now in the presence or absence of ICI antibodies. Then, Tfh cell frequency and function were assessed by flow cytometry. In addition, sorted human Tfh cells were assessed by flow cytometry for proliferation and cytokine/chemokine production following treatment with ICI antibodies.

Results ICI treatment significantly increased Tfh differentiation from naïve human CD4⁺ T cells, with the greatest increase seen with combination anti-PD1 + anti-CTLA4 (p<0.0001). Notably, combination ICI therapy is associated with the highest incidence of IRAEs in patients. ICI binding (anti-PD1 and/or anti-CTLA4) did not have a significant direct effect on human Tfh cell proliferation or cytokine production (IL-21, CXCL13) in vitro.

We then turned to our mouse model of IRAEs, in which autoimmunity-prone NOD mice treated with ICI develop multi-organ autoimmune infiltrates to test the effect of ICI therapy on Tfh cell frequency and function in vivo.⁷ ICI therapy increased the frequency of IL-21⁺ Tfh cells in vivo compared to isotype control, with the greatest increase seen with combination anti-PD1 + anti-CTLA-4 therapy (p<0.01). In addition, treatment with a JAK1/2 inhibitor prevented differentiation of human Tfh cells in vitro and the expansion of Tfh cells and IRAE incidence during ICI therapy in our mouse model, suggesting a dependence upon JAK/STAT signaling.

Conclusions This study identifies a mechanism by which ICIs promote Tfh cell differentiation and highlights potential therapeutic targets for modulating their expansion to reduce IRAEs in patients.

References

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3. Lechner MG, et al. Clonally expanded, thyrotoxic effector CD8+ T cells driven by IL-21 contribute to checkpoint inhibitor thyroiditis. Sci. Transl Med. 2023;15:0675.

4. Zhu C, et al. Increased frequency of follicular helper T cells in patients with autoimmune thyroid disease. J. Clin. Endocrinol. Metab. 2012;97:943–950.

5. Herati RS, et al. PD-1 directed immunotherapy alters Tfh and humoral immune responses to seasonal influenza vaccine. Nat. Immunol. 2022;23:1183–1192.

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