NK cells and NKT cells in innate defense against viral infections

NK cells contribute to innate defense during certain viral infections, but the mechanisms for their regulation and delivery of antiviral effects are i ncompletely understood. A second NK cell population, from within T cell pop ulations - NKT cells - has a unique potential to initiate cellular effector mechanisms, including those delivered by NK cells, provided that the antig en for their restricted TCR is induced during infection. If elicited, parti cular innate cytokine responses promote activation of NK cell cytotoxicity or IFN-gamma production. These responses can contribute to defense by media ting antiviral and/or immunoregulatory effects. Roles of positive or negati ve receptors for target cells in protection against viruses are less clear. Exciting new data indicate that, in at least one system, NK cell receptors that positively signal for activation participate in the recruitment of th ese cells into antiviral defense mechanisms. Other recent evidence suggests that NKT cells may be important for protection during one viral infection and may be artificially activated by delivery of antigen to promote antivir al defense. Taken together, these recent advances in the characterization o f the NK and NKT cell responses are filling in the details of the complex a nd critical events taking place, at the earliest times after challenge, to promote resistance to viruses.