Several kinds of thymic cells express MHC class II antigens, including human-leukocyte-associated antigen-DR (HLA-DR) during postnatal development. The present study was focused on the detection and analysis of HLA-DR immunoreactivity in human fetal thymuses (6-7th month of gestation). Using monoclonal antibodies, indirect immunoperoxidase staining (IIP), immunogold electron microscopy (IGEM), enzyme-linked immunosorbent assay (ELISA) and flow cytometry, HLA-DR immunopositive (IP) thymic cells were found in samples studied. IIP and IGEM demonstrated the presence of HLA-DR IP stromal cells (SCs): epithelial cells (ECs), dendritic-like cells (DCs) and macrophages (MCs) as well as HLA-DR IP lymphocytes (Lys) in all thymic regions. HLA-DR immunoreactivity was more prominent in the medullary ECs (mECs) than in the cortical ECs (cECs). Strong staining of Hassall's corpuscles and the adjacent mECs was seen. The differences in the intracellular distribution of HLA-DR molecules were detailed by IGEM as a first attempt to analyse HLA-DR IP cells at ultrastructural level. ELISA data and two-colour flow cytometric analysis revealed the presence of HLA-DR IP and HLA-DR/CD3 double IP Lys in accordance with the immunocytochemical assays. The results presented enrich the information about HLA-DR IP components of the thymic microenvironment in developing human thymus and raise the question of their role during prenatal T cell differentiation and selection processes.

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These data are similar to those established for the age-dependent changes of antibodies towards exogenous antigens, suggesting that the 'naturally occurring' antibodies against human spermatozoa are not auto-/isoantibodies.

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The genital Chlamydia infection may play a role in the induction of ASA. This is probably a result of the inflammatory process, but not of cross-reactivity between sperm and Chlamydia trachomatis antigens.