Macrophages lacking CD36 (right) take up less S. aureus (green).

The scavenger receptor CD36 is required for binding and internalization of Staphylococcus aureus, report Stuart et al. (page 477). After recognizing lipoteichoic acid on the bacterial surface, CD36 teams up with toll-like receptors, TLR2/6, to trigger expression of pro-inflammatory cytokines.

Researchers already knew that CD36 recognized apoptotic cells in both mammals and flies. In a large-scale RNAi screen in Drosophila, Stuart et al. saw evidence that CD36 was necessary for S. aureus internalization; they then looked for a similar function in mammals. CD36 was necessary for S. aureus engulfment, but not E. coli uptake, and mutations in the cytoplasmic tail of CD36 disrupted bacterial phagocytosis. Although some cytokine expression was detected in the absence of CD36, cotransfection of CD36 and TLR2/6 demonstrated a synergistic effect in the presence of S. aureus. Mice lacking CD36 were unable to clear S. aureus infection.

Stuart et al. show that CD36-mediated internalization of S. aureus boosts downstream TLR-induced responses, and the team hypothesizes that the TLRs' affinity for lipoteichoic acid may increase upon bacterial uptake into the phagosome. Other scavenger receptors are known to interact with bacteria and with TLRs and, like CD36, may modulate TLR signaling, which would otherwise behave as a simple on–off switch.