T Follicular Regulatory Cell–Derived Fibrinogen-like Protein 2 Regulates Production of Autoantibodies and Induction of Systemic Autoimmunity
Waradon Sungnak, Allon Wagner, Monika S. Kowalczyk, Lloyd Bod, Yoon-Chul Kye, Peter T. Sage, Arlene H. Sharpe, Raymond A. Sobel, Francisco J. Quintana, Orit Rozenblatt–Rosen, Aviv Regev, Chao Wang, Nir Yosef, Vijay K. Kuchroo
Abstract
Abstract T follicular regulatory (TFR) cells limit Ab responses, but the underlying mechanisms remain largely unknown. In this study, we identify Fgl2 as a soluble TFR cell effector molecule through single-cell gene expression profiling. Highly expressed by TFR cells, Fgl2 directly binds to B cells, especially light-zone germinal center B cells, as well as to T follicular helper (TFH) cells, and directly regulates B cells and TFH in a context-dependent and type 2 Ab isotype–specific manner. In TFH cells, Fgl2 induces the expression of Prdm1 and a panel of checkpoint molecules, including PD1, TIM3, LAG3, and TIGIT, resulting in TFH cell dysfunction. Mice deficient in Fgl2 had dysregulated Ab responses at steady-state and upon immunization. In addition, loss of Fgl2 results in expansion of autoreactive B cells upon immunization. Consistent with this observation, aged Fgl2−/− mice spontaneously developed autoimmunity associated with elevated autoantibodies. Thus, Fgl2 is a TFR cell effector molecule that regulates humoral immunity and limits systemic autoimmunity.