Expression of CD13 by tumor vasculature. (A) Immunohistochemical analysis of CD13 expression within the lymphomatous component of diagnostic brain biopsy sample of an enrolled patient. Staining was performed by using the anti-CD13 monoclonal antibody SP187 alone (brown signal, 400×). (B) Immunohistochemical analysis of CD13 and αSMA (a marker of pericytes). The costaining was performed with the anti-CD13 monoclonal antibody SP187 (brown) and the anti-αSMA monoclonal antibody 1A4 (red). Black arrows indicate CD13-positive vessels; red arrows indicate αSMA-positive perivascular cells (scale bar, 20 µm; 630×). Left panels: representative photograph of areas with large vessels with pericyte coverage (red) and some microvessels, showing CD13 staining (brown) also in the absence of pericytes. Right: representative photograph of an area with several CD13-positive structures, likely corresponding to microvessels (brown). (C-D) Confocal immunofluorescence analysis of a tissue section stained with a polyclonal anti-CD13 antibody (green) and with the anti-αSMA antibody 1A4 (red) (400×; scale bar, 50 µm). Inset: Three-dimensional projection of CD13 and αSMA staining of a mature vessel (asterisk) (400×; scale bar, 25 µm) showing that CD13 was expressed on the luminal side of the vessels.