NK cells

NK cells represent the third type of lymphocytes, along with T and B lympho cytes. They can be defined morphologically, phenotypically and functionally . Morphologically, they are large granular lymphocytes, like cytotoxic CD8( +) T lymphocytes. Phenotypically, they are characterized by the absence of receptors for antigens and by the presence of a low affinity receptor for t he Fc fragment of IgG, CD16, and an isoform of a neuronal adhesion molecule N-CAM, CD56. Functionally, they are cytotoxic cells able to lyse antibody- coated cells by antibody-dependent cytotoxicity, and can also lyse virus-in fected cells or tumour cells, for example, by a mechanism of natural cytoto xicity. A new family of receptors, expressed on NK cells and a subpopulatio n of memory T cells, ensure negative regulation of cytotoxicity and the cyt okine production of these cells. These receptors recognize conventional and nonconventional MCH class I molecules. They belong either to the superfami ly of immunoglobulins and are called KIR, or to the superfamily of type C l ectins and consist of CD94-NKG2 heterodimers. After recognition of their li gand, these receptors are phosphorylated on a tyrosine residue present in t heir intracytoplasmic part within a motif called ITIM (Immunoreceptor tyros ine-based inhibition motif). Phosphorylation of this motif allows recruit m ent of tyrosine phosphatase SHP-1 and/or SHP-2 proteins, responsible, via t heir enzymatic activity for the inhibitory property of these receptors. The study of the in vivo role of NK cells in the elimination of tumours and mi crobiological infections requires more thorough investigation of the repert ory and functionality of these receptors for class I molecules.