Th1 cells express Tfh effector molecules and associate with B cells at the T-B border following viral infection

Antibodies protect the host from pathogens by neutralizing viruses, targeting pathogens for phagocytosis and complement deposition, and by activating innate immune cells. Most studies on the cellular mechanisms of antibody induction focus on the role of germinal centers (GCs) – areas within lymph nodes (LNs) and spleen where activated B cells undergo mutation and then selection by T follicular helper (Tfh) cells to produce high affinity antibodies. However, specific and long-lived antibodies can be produced in the absence of GCs and Tfh cells, sometimes referred to as “extrafollicular” (EF) responses. However, EF responses are typically assumed to be short-lived and low affinity. We characterized the CD4 ⁺ T cells present in wild type and Tfh cell-deficient mice in lung draining LNs following influenza A virus or SARS-CoV-2 infection to compare the nature of T cell help for activated B cells. We found that a population of helper Th1 cells remains in LNs seven days following viral infection and interacts with IgG2c ⁺ B cells at the T-B border in both WT and Tfh-deficient mice. We propose that Th1 cells promote virus-specific antibody production in parallel with Tfh cells in the GC and that such a response is contiguous with the T-B border and therefore is not an EF response as classically defined.