Background: We have previously shown increased expression of the CD4⁺ cell chemoattractant IL-16 at sites of airway allergic inflammation. Little is known about the significance of IL-16 in allergic inflammation and its role in allergen-driven T-cell cytokine responses. Because IL-16 interacts specifically with CD4⁺ T cells, we hypothesized that IL-16 released at sites of inflammation may modulate the pattern of cytokines produced by CD4⁺ T cells. Objective: We investigated the effects of exogenous rhIL-16 on cytokine production of PBMCs from atopic and nonatopic subjects in response to antigen and PHA. Methods: Primary cultures of freshly isolated PBMCs from ragweed-sensitive atopic subjects and nonatopic subjects were stimulated with ragweed or PHA in the presence or absence of rhIL-16. Supernatant levels of IL-4, IL-5, and IFN-γ were determined by means of ELISA at different time points between 2 and 6 days. Effects of IL-16 on antigen-induced cellular proliferative responses were determined. Results: No IL-4 protein was detected after antigen stimulation of PBMCs from atopic subjects, whereas significant levels of IL-5 were measured on day 6 (median, 534.9 pg/mL). IL-5 secretion was abolished in PBMC cultures depleted of CD4⁺ cells. The addition of rhIL-16 in antigen-stimulated PBMC cultures significantly reduced the amount of IL-5 released (median, 99.8 pg/mL; P < .001). Detectable levels of IFN-γ (median, 53.3 pg/mL) were identified after antigen stimulation. The addition of rhIL-16 in antigen-stimulated PBMC cultures significantly increased IFN-γ levels (median, 255.6 pg/mL; P < .05). Effects of rhIL-16 appear to be specific for antigen-stimulated PBMCs in atopic subjects because rhIL-16 did not alter IL-5 or IFN-γ production in response to PHA nor did rhIL-16 alter cytokine production in nonatopic normal subjects. Conclusion: These studies suggest that IL-16 can play a role in regulating the production of cytokines seen in allergic states in response to antigen. (J Allergy Clin Immunol 2001;107:477-82.)