Innate immune activation during Salmonella infection initiates extramedullary erythropoiesis and splenomegaly

Systemic Salmonella infection commonly induces prolonged splenomegaly in murine or human hosts. Although this increase in splenic cellularity is often assumed to be due to the recruitment and expansion of leukocytes, the actual cause of splenomegaly remains unclear. We monitored spleen cell populations during Salmonella infection and found that the most prominent increase is found in the erythroid compartment. At the peak of infection, the majority of spleen cells are immature CD71^-Ter119⁺ reticulocytes, indicating that massive erythropoiesis occurs in response to Salmonella infection. Indeed, this increase in RBC precursors corresponded with marked elevation of serum erythropoietin (EPO). Furthermore, the increase in RBC precursors and EPO production required innate immune signaling mediated by Myd88/TRIF. Neutralization of EPO substantially reduced the immature RBC population in the spleen and allowed a modest increase in host control of infection. These data indicate that early innate immunity to Salmonella initiates marked splenic erythropoiesis and may hinder bacterial clearance.