Abstract
Pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC, also called CbpA) are major virulence factors of Streptococcus pneumoniae (Spn). These surface-exposed choline-binding proteins (CBPs) function independently to inhibit opsonization, neutralize antimicrobial factors, or serve as adhesins. PspA and PspC both carry a proline-rich domain (PRD) whose role, other than serving as a flexible connector between the N-terminal and C-terminal domains, was up to this point unknown. Herein, we demonstrate that PspA binds to lactate dehydrogenase (LDH) released from dying host cells during infection. Using recombinant versions of PspA and isogenic mutants lacking PspA or specific domains of PspA, this property was mapped to a conserved 22-amino-acid nonproline block (NPB) found within the PRD of most PspAs and PspCs. The NPB of PspA had specific affinity for LDH-A, which converts pyruvate to lactate. In a mouse model of pneumonia, preincubation of Spn carrying NPB-bearing PspA with LDH-A resulted in increased bacterial titers in the lungs. In contrast, incubation of Spn carrying a version of PspA lacking the NPB with LDH-A or incubation of wild-type Spn with enzymatically inactive LDH-A did not enhance virulence. Preincubation of NPB-bearing Spn with lactate alone enhanced virulence in a pneumonia model, indicating exogenous lactate production by Spn-bound LDH-A had an important role in pneumococcal pathogenesis. Our observations show that lung LDH, released during the infection, is an important binding target for Spn via PspA/PspC and that pneumococci utilize LDH-A derived lactate for their benefit in vivoIMPORTANCEStreptococcus pneumoniae (Spn) is the leading cause of community-acquired pneumonia. PspA and PspC are among its most important virulence factors, and these surface proteins carry the proline-rich domain (PRD), whose role was unknown until now. Herein, we show that a conserved 22-amino-acid nonproline block (NPB) found within most versions of the PRD binds to host-derived lactate dehydrogenase A (LDH-A), a metabolic enzyme which converts pyruvate to lactate. PspA-mediated binding of LDH-A increased Spn titers in the lungs and this required LDH-A enzymatic activity. Enhanced virulence was also observed when Spn was preincubated with lactate, suggesting LDH-A-derived lactate is a vital food source. Our findings define a role for the NPB of the PRD and show that Spn co-opts host enzymes for its benefit. They advance our understanding of pneumococcal pathogenesis and have key implications on the susceptibility of individuals with preexisting airway damage that results in LDH-A release.