Immunohistological expression of interleukin 16 in human tonsils

Background: Interleukin 16 (IL-16) acts highly chemotactic on CD4-bearing c ells. Besides chemotaxis, IL-16 has numerous immunomodulatory effects, and not only on T cells. Objective: To determine IL-16 expression in human tonsils. Methods: Tonsillar follicles were immunohistologically characterized to eli cit a possible cellular source of IL-16 expression. Results: The mantle zone of immature and mature B cells was CD22 immunoreac tive (ir), whereas the germinal center of activated B cells was CD23-ir. Pl asma cells that were CD38-ir were observed extrafollicularly beneath the ep ithelium and within the germinal center. T cells were found most frequently in the extrafollicular space, with a majority of CD4 cells. CD68-ir macrop hages were predominantly found within the germinal center. Immunostaining o f anti-IL-16 revealed strong cytoplasmatic reactivity of extrafollicular ce lls and of cells at the outer rim of the mantle zone. Numerous cells adhere nt to the stratified squamous epithelium were IL-16-ir as well. Double immu nostaining identified CD4(+) T cells as the major cellular source of IL-16 expression. Furthermore, a population of CD22(+) B cells at the outer rim o f the mantle zone expressed IL-16 as well. Conclusions: Interleukin 16 was mainly expressed in a typical CD4-like patt ern in human tonsils. Our data strongly suggest that CD4(+) lymphocytes con stitute the major cellular source for IL-16. We hypothesize that the double -immunostained CD4-ir and IL-16-ir cells represent activated T cells. Becau se CD22(+) B cells at the outer rim of the mantle zone expressed IL-16 as w ell, we conclude that this area might constitute the locus of IL-16-mediate d B-cell differentiation.