Expression and functions of innate pattern recognition receptors in T and B cells

Pattern recognition receptors (PRRs) recognize and respond to evolutionarily conserved microbe-specific molecular patterns and therefore play a critical role in host defense towards pathogenic organisms. The current dogma is that the expression and activation of PRRs in innate immune cells serves as an essential link between the innate and adaptive arms of immunity. This has been demonstrated in studies on PRRs, in particular the toll-like receptors (TLRs) and C-type lectins, which revealed that innate recognition of pathogens results not only in the expression of innate response genes, to control or eliminate infectious agents, but also in the expression of chemokines and cytokines that promote and shape the acquired immune reaction. Recent reports of TLR and other PRR expression and the presence of functional TLR pathways in B and T cell subpopulations indicate that PRR signaling could also induce or modulate adaptive immune responses directly, suggesting that cytokine production and survival/proliferation of activated B and T cells could be finely tuned in a pathogen-specific manner. This review analyzes the available evidence on the expression and possible functions of TLR and other PRRs in adaptive immune cells.