Immunohistochemical identification of macrophages and various dendritic cells in paraffin-embedded biopsy material

Monoclonal antibodies (MoAbs) to monocytes/macrophages usually detect surface membrane antigens that are reactive only on cryostat sections of unfixed tissue. Such sections do not afford structural preservation conducive to satisfactory morphological characterization of various cell types. Moreover, in pathology laboratories the indication for immunohistochemical analyses often arises after routine preparation of the biopsy material has been carried out. Another difficulty is that the available MoAbs have shown a considerable degree of detection heterogeneity, including various dendritic cells (DC) and also various cell types beyond the monocyte-derived lineage. Conversely, the L1 protein is a formalin-resistant myelomonocytic antigen for which a MoAb is now available; and this cytoplasmic marker has a remarkably restricted distribution within the monocyte-derived cell lineage. By paired staining for L1 and S-100 protein, macrophages and DC of the interdigitating or Langerhans type can be visualized simultaneously in routine formalin-fixed paraffin sections. Such immunohistochemical staining is useful for the mapping of reactive tissue elements and characterization of histiocytic neoplasms. Interdigitating and Langerhans' DC can, in addition, be demonstrated in paraffin sections of ethanol-fixed tissue by means of MoAb to HLA-DR. MoAbs are also available for detection of follicular DC in such material. Visualization of the latter cells is of interest for the evaluation of lymph nodes from patients infected with the AIDS virus.