Invariant NKT cells are important in the activation and regulation of immune responses. They can also function as CD1d-restricted killer cells. However, the role of activating innate NK cell receptors expressed on NKT cells in triggering cytolytic function is poorly characterized. Here, we initially confirmed that the cellular stress-ligand receptor NKG2D is expressed on CD4− NKT cells, whereas most CD4+ NKT cells lack this receptor. Interestingly, NKG2D+ NKT cells frequently expressed perforin, and both NKG2D and perforin localized at the site of contact with NKG2D ligand-expressing target cells. CD4− NKT cells degranulated in response to NKG2D engagement in a redirected activation assay independently of stimulation via their invariant TCR. NKT cells killed P815 cells coated with anti-NKG2D mAb and CD1d-negative K562 tumor target cells in an NKG2D-dependent manner. Furthermore, NKG2D engagement co-stimulated TCR-mediated NKT cell activation in response to endogenous CD1d-presented ligands or suboptimal levels of anti-CD3 triggering. These data indicate that the CD4− subset of human NKT cells can mediate direct lysis of target cells via NKG2D engagement independently of CD1d, and that NKG2D also functions as a co-stimulatory receptor in these cells. NKG2D thus plays both a direct and a co-stimulatory role in the activation of NKT cells.

Invariant CD1d-restricted NKT cells play important roles in regulating both innate and adaptive immunity. They are targeted by HIV-1 infection and severely reduced in number or even lost in many infected subjects. Here, we have investigated the characteristics of NKT cells retained by some patients despite chronic HIV-1 infection. NKT cells preserved under these circumstances displayed an impaired ability to proliferate and produce IFN-c in response to CD1d-restricted lipid antigen as compared with cells from uninfected control subjects. HIV-1 infection was associated with an elevated expression of the inhibitory programmed death-1 (PD-1) receptor (CD279) on the CD4 À subset of NKT cells. However, blocking experiments indicated that the functional defects in NKT cells were largely PD-1-independent. Furthermore, the elevated PD-1 expression and the functional defects were not restored by anti-retroviral treatment, and the NKT cell numbers in blood did not recover significantly in response to treatment. The functional phenotype of NKT cells in these patients suggests an irreversible immune exhaustion due to chronic activation in vivo. The data demonstrate a severe functional impairment in the remaining NKT-cell compartment in HIV-1-infected patients, which limits the prospects to mobilize these cells in immunotherapy approaches in patients.Key words: CD1d . HIV . Human . NKT cells . Programmed death-1 IntroductionNKT cells are unconventional T lymphocytes that operate on the border between the innate and the adaptive immune systems, and have some characteristics of both systems [1][2][3]. Their invariant TCR recognizes lipid and glycolipid antigens in complex with CD1d molecules expressed primarily on DC, monocytes and B lymphocytes [4][5][6]. In line with their capacity to influence both innate and adaptive immunity, they are believed to be important regulatory cells in diverse settings of autoimmunity [7], cancer [8], allergy [9] and infectious diseases [10]. The observation that NKT cells are efficiently targeted and lost in HIV-1 infection was therefore of significant interest [11][12][13], as their loss may play a role in the severe immune dysregulation and chronic immune activation that is characteristic of this infection [14][15][16][17][18].Human NKT cells have a constitutive memory T-cell-like phenotype, which includes expression of HIV-1 co-receptors CCR5 and CXCR4 [11,13,19], and direct infection may be 902a primary cause of their loss in infected subjects. However, there are several subsets of NKT cells that differ in their expression of surface receptors. CD4 functions as a co-receptor in NKT cells [20,21], and also defines a functionally distinct subset that usually represents roughly half of cells [22][23][24]. Although the CD4 1 subset is more susceptible to HIV-1 infection in vitro and probably also in vivo [11,13], loss of NKT cells in patients may not be restricted to the CD4 expressing cells [11,12,25]. The recovery of NKT cells in patients starting anti-retroviral treatment (ART) is probably ...

The ex-vivo Th1 responses of circulating NKT cells to CD1d-glycolipid complexes are impaired in HIV-infected patients. NKT cell functions may be progressively lost over time in HIV infection, and CD161 is implicated in the regulation of NKT cell responsiveness.