Continual monitoring of intraepithelial lymphocyte immunophenotype and clonality is more important than snapshot analysis in the surveillance of refractory coeliac disease.

OBJECTIVE: An aberrant immunophenotype and monoclonality of intraepithelial lymphocytes (IELs) are frequently found in refractory coeliac disease (RCD). However, the utility of continual monitoring of IEL immunophenotype and clonality in the surveillance of RCD remains to be studied. DESIGN: The diagnostic and follow-up biopsies from 33 patients with CD, 7 with suspected RCD, 41 with RCD and 20 with enteropathy-associated T cell lymphoma (EATL) (including 11 evolved from RCD) were investigated by CD3epsilon/CD8 double immunohistochemistry and PCR-based clonality analysis of the rearranged T cell receptor (TCR) genes. RESULTS: An aberrant immunophenotype (CD3epsilon(+)CD8(-) IELs > or =40%) and monoclonality were detected occasionally in CD biopsies, either transiently in patients with CD not compliant with a gluten-free diet or in those who subsequently developed suspected RCD, RCD or EATL. In contrast, the aberrant immunophenotype and monoclonality were found in 30 of 41 (73%) and 24 of 37 (65%) biopsies, respectively, at the time of RCD diagnosis. Among the patients with RCD who did not show these abnormalities in their diagnostic biopsies, 8 of 10 (80%) and 5 of 11 (45%) cases gained an aberrant immunophenotype and monoclonality, respectively, during follow-up. Irrespective of whether detected in diagnostic or follow-up biopsies, persistence of both abnormalities was characteristic of RCD. Importantly, the presence of concurrent persistent monoclonality and aberrant immunophenotype, especially > or =80% CD3epsilon(+)CD8(-) IELs, was a strong predictor of EATL development in patients with RCD (p=0.001). CONCLUSIONS: Continual monitoring of both immunophenotype and clonality of IELs is more important than snapshot analysis for RCD diagnosis and follow-up, and could provide a useful tool for surveillance of patients at risk of EATL.

Analysis of mutations in immunoglobulin heavy chain variable region genes of microdissected marginal zone (MGZ) B cells suggests that the MGZ of human spleen is a reservoir of memory B cells.

The splenic marginal zone (MGZ), which surrounds the mantle zone (MTZ) in human splenic white pulp, contains a phenotypically and morphologically distinct population of B cells. The origin of MGZ B cells is uncertain. Whereas some experiments in rodents have suggested that they are a distinct cell lineage responsible for the immune response to T-independent type 2 antigens, others have suggested that they are memory B cells derived from a germinal center (GC) response. The progeny of a GC reaction is expected to have rearranged immunoglobulin (Ig) genes that are mutated. The distribution of mutations would be expected to reflect the selection of Ig by its affinity for antigen. We have analyzed rearranged Ig heavy chain variable region (VH) 6 and VH 4.21 genes in MGZ and MTZ B cells microdissected from frozen sections of human spleen to determine whether these genes have the properties of an affinity-selected memory B cell population. MTZ B cells contained germline Ig VH genes, confirming previous reports and providing an internal control for mutational analysis. MGZ B cells contained Ig VH genes that were mutated, showing that these cells had been subjected to a mutational mechanism characteristically active in the GC. The rearranged VH 6 genes showed patterns of mutation indicative of an antigen selection process, whereas the distribution of mutations in VH 4.21 genes was not characteristic of gene selection by conventional T-dependent antigen. Our studies provide the first evidence that the human splenic MGZ is a reservoir of memory B cells.

A20 deletion is associated with copy number gain at the TNFA/B/C locus and occurs preferentially in translocation-negative MALT lymphoma of the ocular adnexa and salivary glands.

The genetic basis of MALT lymphoma is largely unknown. Characteristic chromosomal translocations are frequently associated with gastric and pulmonary cases, but are rare at other sites. We compared the genetic profiles of 33 ocular adnexal and 25 pulmonary MALT lymphomas by 1 Mb array-comparative genomic hybridization (CGH) and revealed recurrent 6q23 losses and 6p21.2-6p22.1 gains exclusive to ocular cases. High-resolution chromosome 6 tile-path array-CGH identified NF-kappaB inhibitor A20 as the target of 6q23.3 deletion and TNFA/B/C locus as a putative target of 6p21.2-22.1 gain. Interphase fluorescence in situ hybridization showed that A20 deletion occurred in MALT lymphoma of the ocular adnexa (8/42=19%), salivary gland (2/24=8%), thyroid (1/9=11%) and liver (1/2), but not in the lung (26), stomach (45) and skin (13). Homozygous deletion was observed in three cases. A20 deletion and TNFA/B/C gain were significantly associated (p<0.001) and exclusively found in cases without characteristic translocation. In ocular cases, A20 deletion was associated with concurrent involvement of different adnexal tissues or extraocular sites at diagnosis (p=0.007), a higher proportion of relapse (67% versus 37%) and a shorter relapse-free survival (p=0.033). A20 deletion and gain at TNFA/B/C locus may thus play an important role in the development of translocation-negative MALT lymphoma.

DDT residues in an east coast estuary: a case of biological concentration of a persistent insecticide.

DDT residues in the soil of an extensive salt marsh on the south shore of Long Island averaged more than 13 pounds per acre (15 kilograms per hectare); the maximum was 32 pounds per acre (36 kilograms per hectare). A systematic sampling of various organisms from the vicinity showed concentrations of DDT increasing with trophic level through more than three orders of magnitude from 0.04 part per million in plankton to 75 parts per million in a ring-billed gull. Highest concentrations occurred in scavenging and carnivorous fish and birds, although birds had 10 to 100 times more than fish. These concentrations approach those in animals dying from DDT poisoning, which suggests that many natural populations in this area are now being affected, possibly limited, by DDT residues. Similar concentrations have been reported elsewhere in North America.

Ongoing mutation in MALT lymphoma immunoglobulin gene suggests that antigen stimulation plays a role in the clonal expansion.

Indirect antigenic stimulation by H. pylori-specific T cells is implicated in the development of low-grade gastric lymphoma of mucosa-associated lymphoid tissue (MALT), however, the role of direct antigen stimulation is unknown. To study the role of direct antigen stimulation in MALT lymphomagenesis and its relationship with the pathogenesis of distinct pathological lesions, which represent different stages of the tumour progression, we cloned and sequenced the rearranged immunoglobulin (Ig) heavy chain gene in three low-grade (two from the lung, one from the stomach) and one high-grade (from the stomach) cases. In the low-grade gastric case, we studied the Ig sequence in primary as well as its disseminated and recurrent tumours. In the high-grade gastric case, we analysed the Ig sequence in tumour cell populations microdissected from the residual diffuse low-grade lesions, diffuse high-grade areas from follicles colonized by high-grade blasts. Compared with the published germline sequences, the heavy chain variable (VH) genes of three MALT lymphomas, in which the putative germline was identified, contained frequent somatic mutations, showing a much higher ratio of replacement/silent mutations in the complementarity determining regions (CDRs) than the framework regions (FRs). Ongoing mutation as indicated by intraclonal variation of the Ig sequence clearly existed in low-grade tumour including its dissemination and recurrence, but was not evident in high-grade tumour cell populations including those microdissected from independent colonized follicles. In addition, the germlines of VH genes used by the three MALT lymphomas are frequently found in autoreactive antibodies. Our results suggest that MALT lymphoma derives from postgerminal centre memory B cells, possibly autoreactive B cell clones, and that direct antigen stimulation may play an important role in the clonal expansion of low-grade MALT lymphoma.

Histological classification of the non-Hodgkin's lymphoma.

The non-Hodgkin's lymphomas (NHL) are a diverse group of neoplasms which show subtle histological differences. This has led to difficulties in formulating reproducible classifications of NHL that are both histologically and clinically relevant. In this review we discuss some of the historical aspects of NHL classification leading up to the principal classifications in current use. In comparing these we put forward reasons why we regard the updated Kiel classification as that which is the most soundly based in clinical, histological and biological terms. The Kiel classification, unlike its competitors, lends itself to further updating as new clinicopathological entities become established, without loss of its essentially sound foundations. Thus the classification of T-cell lymphomas can only be regarded as provisional and certain extranodal lymphomas, which are clear clinicopathological entities, await classification. The biological foundation of the Kiel classification should allow further updating to incorporate these entities and the tighter definitions of categories of NHL that will surely result from the application of cytogenetics, molecular biology and studies of lymphocyte homing mechanisms.

Primary central nervous system lymphoma.

Primary central nervous system lymphomas (PCNSL) are uncommon neoplasms accounting for less than 2% of brain tumours. Their incidence appears to be increasing across a wide age range, in both immunocompetent and immunosuppressed populations. Particular risk groups include those with congenital and acquired immunodeficiencies and transplant recipients. The spread of the AIDS epidemic has seen large numbers of complicating PCNSL develop. Epstein-Barr virus infection appears to play a role in the development of these lymphomas in the immunosuppressed population. The aetiology of these tumours in the immunocompetent is uncertain. Their tendency to remain within the nervous system is not well understood but may be a function of CNS binding molecules carried by lymphocytes. Clinically PCNSL may present with a wide variety of signs and symptoms and has a capacity to mimic many other neurological conditions. Radiologically they appear as hyperdense homogenous deposits in subcortical white matter. Although most lesions are intermediate or high grade B cell lymphomas, T cell lymphomas are being recognised with increasing frequency. Immunohistochemistry and genotypic analysis have an important role in accurately characterising PCNSL, particularly in stereotactic biopsies. Involvement of multiple areas of the neuraxis, the eye and multiple intracranial sites can occur in the absence of obvious systemic lymphoma. The role of surgery in their treatment is uncertain. A combination of radiotherapy and chemotherapy can increase the length of survival. The prognosis, however, remains poor in comparison with nodal lymphomas, and particularly so in those with AIDS.

Mucosa-associated lymphoid tissue lymphoma.

A distinctive type of low-grade extranodal lymphoma recapitulates the cytomorphologic features of mucosa-associated lymphoid tissue (MALT). Typically, these MALT lymphomas arise from sites normally devoid of lymphoid tissue, but are preceded by chronic inflammatory, usually autoimmune, disorders that result in the accumulation of lymphoid tissue. The stomach is the most common site of MALT lymphoma, which arises from lymphoid tissue acquired as the result of Helicobacter pylori infection. The indolent clinical behavior of gastric MALT lymphoma coupled with certain histologic features suggests that its growth is subject to immunologic stimuli, and the role of H pylori in this respect has been examined in detail. In vitro experiments have shown that the growth of lymphoma cells is stimulated by contact with T cells, which, in turn, show strain specific responses to heat-killed H pylori. Clinically, approximately 70% of cases of stage IE gastric MALT lymphoma regress following eradication of H pylori with antibiotics. Large, deeply invasive tumors and those that have undergone high-grade transformation typically do not respond to antibiotic therapy. Other common sites of MALT lymphoma include the salivary glands, lung, and ocular adnexa. The clinicopathologic features of these lymphomas are remarkably similar to gastric MALT lymphoma, which suggests that they, too, may be antigen-driven.

Crypt cell carcinoma of the appendix (so-called adenocarcinoid tumor).

A series of 16 carcinoid tumors of the appendix, including four examples of so-called adenocarcinoid tumors, has been studied immunohistochemically for the presence of lysozyme, secretory component (SC) and IgA, within tumor cells. Stains for mucin, Paneth cells, and APUD cells were also performed. Of the conventional carcinoid tumors, eight showed focal tubule formation with production of PAS-positive material. In contrast to conventional carcinoids, including those showing tubule formation, adenocarcinoids contained few APUD cells and showed positive staining of tumor cells for lysozyme, SC, and IgA. Paneth cells, staining positively for lysozyme, were present in two cases and mucin-containing Paneth cells were observed. A parallel study of normal small intestinal mucosa demonstrated a population of lysozyme containing goblet cells within the crypts. It is suggested that so-called adenocarcinoid tumors are not derived from APUD cells but from lysozyme-producing cells of the type normally present in small intestinal crypts.

Middle East lymphoma and alpha-chain disease. An immunohistochemical study.

An immunoperoxidase study of the small intestinal mucosa of three patients with alpha-chain disease showed heavy infiltration of the mucosa by plasma cells containing alpha-heavy chain and J-chain but no light chains. An additional band-like and nodular mucosal infiltrate was also present and consisted of cells showing no evidence of cytoplasmic immunoglobulin synthesis. The cells comprising this infiltrate invaded and destroyed intestinal crypts, and immunoperoxidase staining showed them to be sharply distinct from the alpha-chain-containing plasma cells. In two cases of Middle East lymphoma, immunohistochemistry revealed a normal plasma cell population in the lamina propria of the small intestine. These results show that alpha-chain disease can be diagnosed in routine paraffin sections which should permit clarification of its true incidence in Middle East lymphoma. The demonstration of sharp distinction between the two types of mucosal infiltrate in alpha-chain disease is in contrast to previous immunofluorescence results and enables more ready identification of mucosal changes that may be important in the management of the disease.

Recent developments in our understanding of gastric lymphomas.

The histopathologic features of low-grade primary gastric lymphoma recapitulate the structure of Peyer's patches [mucosa-associated lymphoid tissue (MALT)] rather than that of lymph nodes. Transformation of low-grade MALT lymphoma to high-grade disease is well recognized, and it is likely that most high-grade primary gastric lymphomas evolve from low-grade lymphoma of the MALT type and are therefore derived from the same B-cell lineage. Molecular genetic studies of gastric MALT lymphomas have shown that these lymphomas do not share any of the features common to nodal lymphomas but, instead, exhibit a marked increase in the frequency of trisomy 3. Gastric MALT lymphomas also differ from their nodal counterparts with respect to their clinical behavior, which is remarkably favorable. The histologic features of gastric MALT lymphomas suggest that one explanation for their favorable behavior may be that their growth is influenced by antigen. That lymphoma should arise from gastric mucosa is paradoxical, because there is no lymphoid tissue in normal stomach. However, several studies have shown that lymphoid tissue accumulates in gastric mucosa almost exclusively as a consequence of Helicobacter pylori infection and that this lymphoid tissue has MALT characteristics. These findings suggested that H. pylori might provide the antigenic stimulus for the growth of gastric MALT lymphoma. Further evidence for this was the finding of H. pylori in more than 90% of cases of gastric MALT lymphoma. Subsequently, evidence supporting an etiologic role for the organism has steadily accumulated. The incidence of gastric lymphoma is greater in communities with a high prevalence of H. pylori, and a case control study has shown that gastric lymphoma is more common in patients infected with the organism; moreover, the infection precedes the onset of lymphoma. Laboratory studies have shown that the growth of tumor cells from low-grade gastric lymphomas can be stimulated by H. pylori and that the effect is strain-specific and is mediated by contact-dependent help from H. pylori-specific T cells. Parallel clinical studies have shown that cases of low-grade gastric lymphoma, when confined to the mucosa, may regress after eradication of H. pylori from the patient's stomach. It remains to be shown whether deeply penetrating or high-grade tumors will respond in the same way. Other outstanding questions relate to the optimal interval between eradication of H. pylori and final evaluation of the response and to the expected duration of the response. On the basis of these laboratory experiments and clinical findings, it is possible to suggest a scheme for the pathogenesis of gastric MALT lymphoma.

The natural history of benign lymphoepithelial lesion of the salivary gland in which there is a monoclonal population of B cells. A report of two cases.

In two patients, a diagnosis of benign lymphoepithelial lesion of the salivary gland was followed by the development of extrasalivary gland lymphoma after 10- and 9-year intervals, respectively. On review, immunohistochemistry revealed immunoglobulin light-chain restriction in the initial biopsy in each case and there was both morphological and immunohistochemical evidence linking the extrasalivary gland lymphoma with the initial lesion. It is argued that in the presence of a monoclonal B-cell population, a diagnosis of benign lymphoepithelial lesion is inappropriate. These patients fulfill the criteria for a diagnosis of low-grade B-cell lymphoma of mucosa-associated lymphoid tissue and should be treated accordingly.

Demonstration of cytokeratins and an epithelial membrane antigen in chordomas and human fetal notochord.

The application of immunohistochemical staining with anti-epithelial monoclonal antibodies to the differential diagnosis of chordomas is described. Cytokeratins and an epithelial membrane-specific oligosaccharide sequence are found in chordomas but not in chondrosarcomas or normal cartilage. The same cytokeratins and oligosaccharide sequence are demonstrated in human fetal notochord. Immunohistochemical staining with antiepithelial antibodies is therefore of value in distinguishing chordomas from cartilaginous tumours. The staining of notochord with the same monoclonal antibodies adds weight to the proposition that chordomas arise from embryonic rests of notochordal cells.

Follicular colonization in B-cell lymphoma of mucosa-associated lymphoid tissue.

The formation of neoplastic B-cell follicles is universally accepted as diagnostic of a follicle centre cell (FCC) lymphoma. Low-grade B-cell lymphomas of mucosa-associated lymphoid tissue (MALT) are characterized by a diffuse infiltrate of cells of uncertain lineage known as "centrocyte-like" cells because of their resemblance to centrocytes (small cleaved cells). Some MALT lymphomas, however, contain numerous follicles and may even have a predominantly follicular appearance. These follicles may be reactive or show immunoglobulin (Ig) light-chain restriction, indicating their neoplastic nature. We have proposed that these neoplastic follicles are not composed of follicle centre cells but result from colonization of reactive follicles by CCL cells. In this study, the immunophenotype and genotype of 10 primary gastrointestinal lymphomas with a follicular component have been determined. One case exhibited the morphological, immunophenotypic, and genotypic features of FCC lymphoma (Ig light-chain restriction, CD10+, KB61 (CDw32)-, Jh, and bcl-2 gene rearrangement). Neoplastic follicles in the remaining nine cases, which showed the features of MALT lymphoma, were of a different phenotype (Ig light-chain restriction, CD10- KB61(CDw32)+), and these lymphomas showed Jh but not bcl-2 gene rearrangement. Taken in conjunction with the morphological features, these findings suggest that in these cases the neoplastic follicles formed as the result of colonization of previously reactive follicles by neoplastic CCL cells. Thus, not all lymphomas containing neoplastic follicles are of FCC origin. Follicular colonization, as seen in low-grade MALT lymphomas, is likely to be a recapitulation of an as yet undescribed normal immunological phenomenon that may involve marginal zone B cells.

Antigen preservation in infarcted lymphoid tissue. A novel approach to the infarcted lymph node using monoclonal antibodies effective in routinely processed tissues.

We studied 11 cases of malignant lymphoma diagnosed concurrently with or following lymph node infarction. Cases included seven B-cell lymphomas, three T-cell lymphomas, and one case of Hodgkin's disease. Sections of viable and infarcted tissue were immunostained in parallel using a panel of antibodies effective in routinely processed, wax-embedded tissue. The panel included anti-leucocyte-common antigen (CD45), T-cell-associated antigens (UCHL1, MT1), B-cell-associated antigens (MB1, 4KB5 (CD45R), MT2, LN1), a B-cell-specific antigen (L26), C3D-1 (CD15), and BER-H2 (CD30). Antibodies to intermediate filament cytoskeletal proteins, epithelial membrane antigen, and Factor VIII-related antigen were also used. In eight cases, staining of the infarcted material gave evidence of a lymphoid proliferation of either T- or B-cell type; an in the case of Hodgkin's disease, the results supported this diagnosis. The immunophenotype derived in the infarcted tissue mirrored the findings in the viable material in these eight cases of non-Hodgkin's lymphoma. A case of testicular infarction with concurrent intraosseous lymphoma was also examined. Staining in this case provided evidence of infarcted lymphoma. Thus, immunostaining of infarcted lymphoid tissue with these novel antibodies provides valuable information that conventional light microscopy cannot offer.

T-cell-rich B-cell lymphoma.

Five cases of B-cell lymphoma are described in which the morphology and initial immunohistochemistry suggested a diagnosis of T-cell neoplasia. In four cases, the histological picture was that of an adult pleomorphic T-cell lymphoma; the fifth case was a lymphocytic lymphoma (CLL) with an accompanying T-cell lymphocytosis in the peripheral blood. Immunohistochemistry on both frozen and paraffin-embedded material showed that the cellular population in all five cases consisted mainly of T-cells; less than 10% of the cells stained as B-cells. However, immunoglobulin immunostaining combined with use of the new lineage-related monoclonal antibodies reactive in paraffin section revealed that the B-cells constituted the neoplastic population. Genetic analysis showed no evidence of clonality in the T-cells, nor was it able to detect rearrangement in the small number of clonal B-cells present. These cases represent a variety of B-cell neoplasia that mimicks T-cell lymphoma morphologically and immunologically. The vigorous T-cell reaction seen in such lymphomas means that the malignant population can be correctly identified only by careful examination of the immunohistochemical findings.

Immunoproliferative small-intestinal disease. An immunohistochemical study.

We performed a detailed histological and immunohistological study on both fresh-frozen and paraffin-embedded tissue from full-thickness jejunal biopsy specimens taken from three patients with immunoproliferative small-intestinal disease (IPSID). In all three patients, the mucosal infiltrate consisted of "centrocyte-like" (CCL) cells forming lymphoepithelial lesions and plasma cells. In one patient, the mucosal infiltrate was strikingly follicular. Immunohistochemistry showed alpha 1 heavy chain, but no light chain, in the perinuclear space and cytoplasm of the CCL cells and in the plasma cells. In two patients, the plasma cells (but not the CCL cells) also contained alpha 2 heavy chain. In the case showing a follicular pattern, the extrafollicular CCL cells and most of the cells within the mucosal follicles expressed alpha 1 heavy chain, but a minor and variable population of cells expressed polytypic IgM. The dendritic reticulum cells stained for alpha 1 (but not alpha 2) heavy chain, mu chain, and both light chains. In all cases, the CCL cells did not stain for common acute lymphoblastic leukaemia antigen (CALLA); in the follicles, CALLA negative cells displaced a residual CALLA-positive population to the periphery and merged with the CALLA negative cells outside the follicles. These findings confirm the homology between IPSID and low-grade B-cell "Western" lymphomas arising in mucosa-associated lymphoid tissue; they suggest that the follicular pattern sometimes seen in these lymphomas is caused by selective colonization of reactive follicles by CCL tumor cells.

Splenic marginal zone cell lymphoma.

We describe four female patients with primary splenic low-grade non-Hodgkin's B-cell lymphomas with the morphology and immunophenotype of splenic marginal zone lymphocytes. The patients presented with splenomegaly, anemia, and weight loss. The bone marrow was involved in all four cases. Liver involvement was found in one patient; and in another, a CT scan revealed lymphadenopathy in the chest and abdomen. The histology of the spleen was characterized by broad concentric strands of monomorphic medium-sized lymphocytes around lymphoid follicles in one case and infiltrating follicles in two cases. Selective replacement of follicles was seen in one case. Tumor in splenic hilar lymph nodes (four cases) and liver (one case) was similar. Three patients remain well 4, 9, and 12 months, respectively, after splenectomy without further treatment. One patient who received chemotherapy died 1 year after splenectomy.

CD10 and BCL-6 expression in paraffin sections of normal lymphoid tissue and B-cell lymphomas.

In this study the authors explored the value of immunostaining for follicular center B-cell markers, BCL-6 and CD10, in paraffin sections as a tool for the differential diagnosis of B-cell lymphomas. The cases studied comprised reactive lymphoid hyperplasia (RLH; n = 19), follicular lymphoma (FL; n = 50), low-grade mucosa-associated lymphoid tissue (MALT) lymphoma (n = 24), mantle cell lymphoma (n = 19), splenic marginal zone lymphoma (n = 13), diffuse large B-cell lymphoma (DLBCL; n = 54), Burkitt's lymphoma (BL; n = 20), nodular lymphocyte predominance Hodgkin's disease (NLPHD; n = 16), and classic Hodgkin's disease (CHD; n = 13). In RLH, CD10 and BCL-6 were expressed almost exclusively by the follicular center cells. In contrast in FL, the expression of CD10 (39/50) and BCL-6 (34/36) was seen in both follicular and interfollicular neoplastic B cells. Marginal zone/MALT lymphomas and mantle cell lymphoma were always negative. In DLBCL the expression was variable for both CD10 (21/54) and BCL-6 (39/47), with some tumors, including cases of transformed follicular lymphoma (9/10), coexpressing CD10 and BCL-6, and others expressing only BCL-6, and a small group expressing neither marker, possibly reflecting the underlying primary pathogenetic events such as the rearrangement of BCL-2 or BCL-6 genes. BL was always both CD10 and BCL-6 positive. In NLPHD the L&H cells expressed BCL-6 (11/13) but not CD10, whereas in CHD BCL-6 expression was seen in half of the cases. This study demonstrates that both CD10 and BCL-6 are reliable markers of follicular center B-cell differentiation. CD10 and BCL-6 immunostaining have an important role in differential diagnosis of FL from RLH and other low-grade B-cell lymphomas. The results also suggest that a CD10/BCL-6 expression pattern may be helpful in identifying main subsets of DLBCL. However, additional studies comparing genotype with immunophenotype are required.

Low-grade B-cell lymphoma of mucosa-associated lymphoid tissue arising in the thymus. A thymic lymphoma mimicking myoepithelial sialadenitis.

We describe two cases of primary low-grade B-cell lymphoma of the thymus that showed histological features of low-grade B-cell lymphoma arising in mucosa-associated lymphoid tissue (MALT). The appearances most closely resembled MALT lymphoma arising in myoepithelial sialadenitis (MESA). In both cases, the tumor was excised. In one case, there has been no recurrence in 4 years of follow-up without further treatment; in the second case, the tumor has involved an axillary lymph node. Immunohistochemistry showed light-chain restriction in both cases, and the B-cell phenotype was similar to that previously described in MALT lymphomas. The occurrence of MALT lymphoma in the thymus is consistent with the presence of mucosal structures (Hassall's corpuscles) and with recent descriptions of a native B-cell population in this organ. The relationship of this previously undescribed thymic low-grade B-cell MALT lymphoma arising in the thymus has not yet been clarified.