The CD6 scavenger receptor is differentially expressed on a CD56 natural killer cell subpopulation and contributes to natural killer-derived cytokine and chemokine secretion.

The CD6 scavenger receptor is known to be expressed on virtually all T cells and is supposed to be involved in costimulation, synapse formation, thymic selection and leukocyte migration. Here, we demonstrate that CD6 is differentially expressed by a subpopulation of peripheral CD56(dim) natural killer (NK) cells and absent on CD56(bright) NK cells. CD56(dim)CD16(+) cells represent the major NK subset in the periphery, and most cells within this group are positive for CD6. Most killer immunoglobulin-like receptor- and immunoglobulin-like transcript-positive cells also belong to the CD6(+) subpopulation, as expected from their restricted expression on CD56(dim) NK cells. In addition, CD6(+) NK cells are similar to the newly identified CD94(low)CD56(dim) NK subpopulation and most distant from the recently defined CD27(+) NK subpopulation based on the reverse correlation of expression between CD6 and CD27, a marker associated primarily with CD56(bright) NK cells. With respect to CD6 function on NK cells, direct CD6 triggering did not result in degranulation but induced secretion of cytokines (interferon-γ and tumor necrosis factor-α) and chemokines [CXCL10 (IP-10), CXCL1 (GRO-α)]. Thus, CD6 expression on peripheral NK cells marks a novel CD56(dim) subpopulation associated with distinct patterns of cytokine and chemokine secretion.

TCR-independent cytokine stimulation induces non-MHC-restricted T cell activity and is negatively regulated by HLA class I.

Recent evidence suggests that the functional status of T cells activated independently from their TCR differs substantially from classical MHC-restricted T cells. Here, we show that TCR-independent, short-term stimulation via the common gamma-chain of the IL-2/IL-15 receptor induces non-MHC-restricted cytotoxicity and sustained cytokine secretion in purified CD4+ or CD8+ T cells. NK-like cytotoxicity is directed against MHC class I-negative targets and can be inhibited by classical and non-classical HLA class I molecules. Known inhibitory receptors, such as CD85j (ILT2) and leukocyte-associated Ig-like receptor-1, are not responsible for this HLA-mediated inhibition. NK-like cytotoxicity can be costimulated by NKG2D (CD314) triggering, but 2B4 (CD244) and DNAM-1 (CD226) are not involved. NK-like T cells display an activated phenotype and secrete various cytokines, including IFN-gamma, TNF-alpha, IL-5, IL-13 and MIP-1beta. Under normal conditions, HLA class I-mediated inhibition may function as a safety mechanism to prevent unbalanced cytokine production and effector killing mechanisms by T cells that were activated independently from their TCR. Non-MHC-restricted activity represents a functional status rather than a property of distinct T cell subpopulations. Thus, cytokine-induced, non-MHC-restricted T cells may be relevant in immune responses against tumors showing aberrant MHC expression through their capacities of cytokine production and direct tumor cell eradication.