The cells of innate and adaptive immunity, although activated by different ligands, engage in cross talk to ensure a successful immune outcome. Toll-like receptors (TLRs) are key components of the innate immune system and have the ability to detect microbial infection and trigger host defence responses. Otherwise, human T lymphocytes are able to produce most TLRs. Thus, we analyze the capability of some TLR ligands to modulate the function of highly-purified CD4+ T cells. We found that agents acting via TLRs (poly I:C, a TLR3 ligand; flagellin, a TLR5 ligand; and R848, a TLR7/8 ligand) are able to regulate the expression of costimulatory molecules both on purified antigen presenting cells and on purified T lymphocytes. Moreover, the activation mediated by TLRs determines a kinetic expression of B7-family members such as through an inhibition of T lymphocytes delayed proliferation. These findings suggest a functional role of some invading microorganisms in regulating acquired immunity
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