IL-16 immunoreactivity is observed confined to T-bet+ cells and active caspase-3+ cells in MS lesions. A) In acute lesions (AL) in MS brain, prominent T-bet+ cells were found in small infiltrates scattered throughout the white matter of parietal lobe. IL-16 immunoreactivity was observed at similar locations within the lesion. DAPI staining indicates that the immunostaining is localized to mononuclear cells. The merged image shows that IL-16 immunoreactivity co-localizes to some T-bet+ cells. The image on the far right shows enlarged detail from the merged image (at arrows), where IL-16 confined to T-bet+ cells (at arrows) can be better appreciated. It also shows a T-bet+ cell, which is IL-16- (arrowhead), and a T-bet-, IL-16- cell (asterisk). B) In an AL of lumbar spinal cord, intense immunostaining for IL-16 and for active caspase-3 is seen in dense perivenular mononuclear cell infiltrates in the white matter. A detail from the merged image (at arrows) is shown in the far right image. Most infiltrating cells exhibit a nuclear pattern of immunostaining for active caspase-3. Some of these active caspse-3+ cells are IL-16+ (at arrows). Note the distinct patterns of IL-16 and active caspase-3 immunostaining. While IL-16 nuclear immunostaining is observed occasionally (asterisk), more often it appears as cytoplasmic or membrane-bound immunostaining, or as secreted product adjacent to cells (at arrows). IL-16 was found frequently between adjoining mononuclear cells (arrowhead). Two color fluorescence × 60.