Activated follicular B helper T-cell phenotype of neoplastic cells in AITL. (A) Double immunostaining for CD10 in brown (DAB) and CXCL13 in red (Fast red TR) shows coexpression of CD10 and CXCL13 in tumor cells (no counterstain). (B) Double immunostaining for CXCL13 in red (Fast red TR) and CXCR5 in brown (DAB). The CXCL13⁺ neoplastic cells display concomitant positivity for CXCR5. (C) Double immunostaining for CXCL13 in red (Fast red TR) and CD154 in brown (DAB) that demonstrates overlaying cytoplasmic staining in the neoplastic cells. (D) Double immunostaining for CD134 in brown (DAB) and CXCL13 in red (Fast red TR) shows coexpression of these 2 antigens in the neoplastic cells. Original magnifications, × 400 (A,C-D) and × 1000 (B). The double stainings were performed using EnVision-HRP (A,D) or CSA II System (B-C) in combination with EnVision-AP (A-D; all from DakoCytomation, Carpinteria, CA). Primary antibodies were as follows: polyclonal goat anti-CXCL13 (A-D) and monoclonal mouse anti-CXCR5 (B; R&D Systems, Minneapolis, MN); monoclonal mouse anti-CD10 (A), anti-CD154 (C), and anti-CD134 (D; Novocastra Laboratories, Newcastle upon Tyne, United Kingdom). Images were visualized on a Nikon Eclipse E600 microscope equipped with a Nikon Plan 40 ×/0.65 objective or 100 ×/1.25 oil-immersion objective lens; images were then captured via a Nikon Coolpix 4500 digital camera and processed with Adobe PhotoShop 7.0 software (Adobe Systems, San Jose, CA).