Significant differences in receptor expression between CD56⁺ immune cell subsets. Model depicting the distribution of receptors on (A) CD4^neg NK cells, (B) CD4⁺ NK cells, and (C) CD56⁺ monocyte/DC-like cells. Conclusions are based on results in Figures 3 and 5. Receptors expressed on the majority of cells of that subset are represented in black. No expression or < 15% of cells expressing a receptor is indicated in gray. Receptor expression in panel C is representative of a heterogeneous population of monocytes and DC-like subsets. For example, CD16 is not expressed on CD123⁺ cells, but is expressed on many of the CD13⁺ cells. The expression of cytokines after stimulation with K562 target cells, IL-12 + IL-18, and poly(I:C) for the particular cell subset is indicated below. Only CD7⁺ NK cells produced IFNγ after stimulation with NK cell–specific stimuli, whereas a strong IL-6 and TNFα response was observed in the CD7^negCD56⁺ cells after poly(I:C) stimulation. The ability of CD7⁺ NK cells and CD7^negCD56⁺ DCs/monocytes to induce proliferation in a mixed leukocyte reaction (MLR function) is also indicated; only CD7^negCD56⁺ cells are inducers of proliferation in accordance with their phenotype as DCs/monocytes.