PS-01-014 Pulmonary nodular lymphoid hyperplasia ("pulmonary pseudolymphoma") -The significance of increased numbers of IgG4 positive plasma cells D. Guinee * , A. Gerbino, S. Murakami, M. Koss * Virginia Mason Medical Center, Pathology, Seattle, USA Objective: Pseudolymphomas of the skin, breast, and lacrimal glands show an increase in IgG4 positive plasma cells. We hypothesized that a similar increase in IgG4 positive plasma cells occurs in pulmonary nodular lymphoid hyperplasia (PNLH). Method: Immunohistochemical stains for IgG4 and IgG were performed in 2 cases of PNLH, 7 cases of BALT lymphoma, 8 cases of intraparenchymal lymph nodes (IPL) and 1 case of follicular bronchiolitis (FB). The mean number of IgG4-and IgG-positive plasma cells and the IgG4/IgG ratio was obtained from a manual count of three separate high power fields (hpfs) of areas of highest cellularity. Results: The average number of IgG4 positive plasma cells and the IgG4/IgG ratio was increased in PNLH (case 1: 225/hpf, ratio: 0.39, case 2: 280/hpf, ratio: 0.59). In comparison, average IgG4 positive plasma cells per hpf were much lower in BALT lymphoma (range=0-2.3/hpf, ratio=0-0.03), IPL (range IgG4/ hpf=0-102, ratio=0-0.23) and FB (1/hpf, ratio=0.03). Conclusion: The increase in IgG4 positive plasma cells and IgG4/IgG ratio in PNLH aids in diagnosis and supports our current understanding of PNLH as a distinct form of reactive lymphoid proliferation. The relationship between PNLH and IgG4-related sclerosing disease requires further study.