2A). Confirming results obtained on total NK cells, expression of KIR2DL1 but not of KIR3DL1 increased on NKG2C+ cells (Fig. 2C and D). Interestingly, a small but statistically significant increase in KIR2DL1 on NKG2C+ was detected also in CMV-seronegative donors; however, this increase was much smaller than that seen in this website CMV-seropositive donors (Fig. 2C). To discriminate between expression of KIR2DL2/S2 and KIR2DL3, we next cultured PBMCs from donors carrying

the genes for all three receptors. Co-staining of a KIR2DL3 specific Ab with an Ab recognizing KIR2DL2/S2/L3 allowed us to distinguish between expression of KIR2DL2/S2 and KIR2DL3 (Fig. 3A). In five CMV-seropositive donors, strong expansion of KIR2DL3-expressing NK cells was documented, while co-culture with

CMV-infected fibroblasts had no impact on the expression of KIR2DL2/S2 (Fig. 3B and C). To address whether the increased expression of KIR-expressing cells represents true expansion, we determined cell number weekly during the 21-day co-culture with MRC-5 in the presence or absence of CMV. The APO866 price NK-cell number contracted during the first week, followed by an expansion of NK cells exclusively in seropositive donors in the presence of CMV (Supporting Information Fig. 2). Staining for the proliferation marker Ki-67 corroborated these results: infection of MRC-5 with CMV led to a massive up-regulation of Ki-67 on NK cells if these stemmed from CMV-seropositive donors (Fig. 2B). Interestingly, when the KIR repertoire was assessed on Ki-67+ cells, we noted expansion of KIR2DL1/Ki-67 double positive but not of KIR3DL1/Ki-67 double positive cells after co-culture with CMV-infected MRC-5 (Fig. 2E and F). We next aimed

to characterize factors PLEK2 influencing the expansion of KIR-expressing NK cells. HLA-C1 group Ags are the ligand for KIR2DL2/S2/L3, while HLA-C2 group Ags are the ligands to KIR2DL1 [17]. If CMV-seropositive donors were stratified according to their KIR ligand status, an expansion of KIR2D-expressing NK cells occurred only in the presence of the cognate KIR ligand: KIR2DL1 expanded only in donors carrying a C2 ligand (Fig. 4A and B), whereas KIR2DL2/S2/L3 NK cells expanded exclusively in the presence of the cognate group C1 ligand (Fig. 4C and D). While no ligand has been identified for the activating KIR receptor KIR3DS1 [18], genetic association studies have suggested an epistatic interaction of KIR3DS1 with HLA-Bw4 in HIV infection [19]. Analysis of Bw4-status in conjunction with KIR3DS1 expression in our population showed that expansion of KIR3DS1 occurred irrespective of the presence of Bw4 (day 21 KIRDS1 expression in CMV-exposed versus CMV nonexposed cells in seropositive donors: mean 23 versus 8% in Bw4-negative, and 31 versus 11% in Bw4-positive donors, p < 0.05 for both comparisons).