Splenic Marginal-Zone Lymphoma: One or More Entities? A Histological, Immunohistochemical and Molecular Study of 41 Cases.

The considerable heterogeneity of splenic marginal-zone lymphoma (SMZL) hinders firm conclusions concerning histogenetic origin and differentiation stage of the neoplastic cells. We analyzed a series of 41 SMZL cases, diagnosed on splenectomy specimens after established WHO criteria, and correlated histological, immunohistochemical and molecular findings in an attempt to gain insight into the origin(-s) of the currently elusive SMZL clonogenic cell(-s). A predominantly nodular growth pattern was observed in 23 cases; the remainder showed predominantly (11 cases) or exclusively (7 cases) diffuse infiltration. Twenty cases showed the “classical” biphasic appearance; the remainder were monophasic, mainly with MZ morphology; 8/41 cases were composed predominantly of small cells. CD5 was expressed in 2/37 analyzed cases; all cases were negative for CD43. CD21 and CD35 were expressed by a minority of cases (11/40 and 16/37 cases, respectively), generally in a concordant fashion (double positive or negative cases). Twenty-two out of 40 analyzed cases were DBA.44-positive; 4/39 evaluable cases co-expressed CD21/DBA.44, while 10/39 cases were double negative. Similar results were obtained for CD35/DBA.44 co-expression. Seventeen out of 37 analyzed cases were SIgD+; 12/22 analyzed cases were SIgM+SIgD+, 7/22 were SIgM+SIgD-, while one case was SIgM-SIgD+; 5/36 analyzed cases were SIgG+. Plasmacytic differentiation was observed in 4/41 cases. Fifteen out of 36 evaluable cases were CD21/SIgD double negative, while 5/36 co-expressed CD21/SIgD. Strong/weak CD27 staining was observed in 22/27 analyzed cases; 8/19 CD27+ cases were SIgD+. Among five CD27-negative cases, 3 were SIgD-positive. Forty IGHV-D-J rearrangements were amplified in 35/41 analyzed cases; double in-frame rearrangements were amplified in three cases. IGHV1/3/4 subgroup genes were used in 8/11/17 sequences. Twenty-five IGHV genes (all IGHV3 and 10/17 IGHV4) were mutated (<98% homology to germline); 5/15 and 7/15 unmutated sequences utilized, respectively, IGHV1 and IGHV4 genes. Two IGHV1-69/IGHD3-16/IGHJ3 rearrangements (one mutated, one unmutated) had homologous HCDR3 regions, after a pattern previously described by several groups in CLL patients (Blood. 2004;104:2499–2504; J Exp Med. 2004;200:519–525; Blood. 2004;104:2879–2885). Clonal IGKV-J and IGLV-J gene rearrangements were amplified in 29/41 cases; 13/29 IGKV/IGLV sequences were mutated. Five out of 25 mutated IGHV genes and 3/13 mutated IGKV genes had high (>3.0) replacement to silent (R:S) mutation ratios in the CDRs along with low (<2.0) R:S ratios in the FRs. Five out of 17 CD27+ cases with available IG data carried unmutated IGHV genes, while 2/5 CD27- cases were IGHV-mutated (both SIgD+DBA.44+). Six SIgD+ cases carried mutated IGHV genes. The results of the present study: confirm the considerable histological, immunohistochemical and molecular heterogeneity of SMZL lymphoma; indicate an origin from the diverse resident B cell populations of the normal splenic marginal zone and do not support a mantle-zone cell derivation; allude to the role of selective antigenic pressures in the pathogenesis of at least a subset of SMZL cases.