Class-biased regulation of humoral immunity and mTORC1 inhibition by physiological hypoxia

Abstract

Abstract B lymphocytes activated by cognate antigen (Ag) proliferate, are selected for affinity maturation, undergo antibody (Ab) class switching dependent on the cytosine deaminase AID, and yield major forms of memory via germinal center (GC) reactions. Although metabolic programming has emerged as a means of mediating or initiating changes in immunity, nothing is known as to the metabolic landscape in lymphoid organs. Here we show that B lymphocytes in GC are hypoxic, and that low oxygen alters B cell Ab response as well as metabolic programming. In addition to increased B cell death, restricted oxygen concentrations impaired Ab class switching by reducing expression of AID and suppressing the pro-inflammatory IgG2c Ab isotype. Prolyl hydroxyl dioxygenases (PHD) and von Hippel-Lindau (VHL) are central to regulation of hypoxia-induced factors (HIF) in hypoxic responses. B cell-intrinsic deficiency of VHL attenuated Ag-specific IgG2c, and decreased levels of Ag-specific GC, early memory B cells, and Ab responses. Hypoxia and VHL deficiency each also inhibited activity of mTORC1 in a manner dependent on HIF proteins. Genetic limitation of mTORC1 in B cells reduced their clonal expansion, AID expression, and IgG2c Ab responses. Collectively, these results provide evidence that GC reactions are sites of physiologic hypoxia that in turn regulates vital functions of B cells through interruption of program that maintains mTORC1 activity. We propose that this restriction of oxygen regulates the balance of fate choices made by GC B cells, and that pathophysiological states that alter oxygen homeostasis may moderate the pro-inflammatory component of humoral immunity.