The prostate-associated lymphoid tissue (PALT) is linked to the expression of homing chemokines CXCL13 and CCL21

BACKGROUND. The genitourinary tract is regarded as part of the mucosal immune system. However, the structural and functional aspects of the human prostate-associated lymphoid tissue (PALT) have never been extensively explored. METHODS. This article describes our investigation of this issue by means of immunohistological, confocal, and ultrastructural examination of the normal human prostate. RESULTS. PALT consists of two main components: (1) intraepithelial leukocytes, namely CD3(+)T cells with prevalent CD8(+) and CD45RA(-)CD45RO(+) phenotype, sometimes CD69(+), followed by CD94(+)NK, CD11c(+)DCs, some expressing CD86, DC-SIGN(+)DCs and a few B lymphocytes; (2) lymphoid aggregates, frequently below the epithelia, arranged in B cell follicles, endowed with a central ICAM-1(+)VCAM-1(+)CD21(+)FDCs network expressing BLC/ CXCL13, and parafollicular T cell areas crossed by PNAd(+)HEV-Iike vessels showing SLC/ CCL21 expression. Parafollicular areas were formed of prevalent CD4+T lymphocytes, both CD45RA(-) and CD45RO(+), and intermingled with CD11c(+)DCs. Germinal-center-containing follicles are few and their parafollicular areas are scantily infiltrated by Foxp3(+)CD69(-) highly suppressive regulatory T cells. Most lymphoid follicles lack a distinct germinal center and their parafollicular area harbor numerous Foxp3(+)CD69(-) cells. CONCLUSIONS. Comparison with the tonsils shows that PALT displays immunomorphological features required for the onset of cellular and humoral immune responses, while its T regulatory cells appear to function as suppressor-regulators of T and B cell responses.