CHARACTERIZATION OF THE LYMPHOID INFILTRATE IN HASHIMOTO THYROIDITIS BY IMMUNOHISTOCHEMISTRY AND POLYMERASE-CHAIN-REACTION FOR IMMUNOGLOBULIN HEAVY-CHAIN GENE REARRANGEMENT

A close relationship between Hashimoto thyroiditis (HT) and low-grade B-cell lymphoma of mucosa;: associated lymphoid tissue (MALT) has been shown. We used immunohistochemistry to study paraffin sections from 4 0 unselected cases of HT and scored cases according to the lymphoid in filtrate and presence of lymphoepithelial lesions (LELs). Clonality wa s assessed by kappa/lambda immunohistochemistry and polymerase chain r eaction for immunoglobulin heavy chain gene rearrangement (IgH PCR). H istologic findings were compared with 2 cases of primary thyroid MALT- type lymphoma. In HT, the lymphoid infiltrate consisted predominantly of T cells in all cases; B cells, associated with germinal centers, di d not have the appearance of marginal zone cells. All cases had identi fiable T-cell LELs; immunohistochemistry confirmed inconspicuous, rare B-cell LELs in 13 of 40 cases. In all cases, plasma cells were polycl onal and IgH PCR showed a polyclonal pattern. Clinical follow-up was a vailable for 34 patients. Lymphoma developed in none. In contrast a B- cell predominant infiltrate of marginal zone cells was present in the MALT-type lymphomas that was not confined to germinal centers. Cytoker atin stains demonstrated severe loss of epithelial elements and destru ctive LELs. LELs are not, in isolation, a useful criterion for disting uishing low-grade MALT-type lymphoma of the thyroid from HI: Features associated with low-grade MALT-type lymphoma include a predominance of B cells, marked loss of epithelial elements, and destructive LELs; co mposed of marginal zone B cells. Unselected cases of HT do not contain monoclones detectable by IgH PCR. Hashimoto thyroiditis (HT) is an au toimmune inflammatory disorder. Histologically it is characterized by lymphocytes infiltrating thyroid parenchyma, reactive germinal centers , and Hurthle cell changes. This inflammatory condition can be conside red a form of acquired mucosa-associated lymphoid tissue (MALT), analo gous to Helicobacter pylori-induced gastritis in the stomach.(1) Previ ous studies have characterized the inflammatory cell infiltrate in HT and found the lymphoid cells to consist of large numbers of T cells.(2 ,3) Recently, a relationship between HT and low-grade B-cell lymphomas of the MALT type has been suggested in part because of histologic sim ilarities.(4,5) We attempted to study this relationship by further des cribing the immunophenotypic, genetic, and clinical features of unsele cted cases with the histologic features of HT. The histologic and immu nophenotypic features of HT were compared with 2 cases of primary thyr oid lymphomas of the MALT type.