Atypical Lymphoid Proliferations

The Pathologist's Viewpoint

Mahmoud Rezk Abdelwahed Hussein


Expert Rev Hematol. 2013;6(2):139-153. 

In This Article

Atypical Lymphoid Proliferations

AtLP are conditions in which it is not possible to differentiate between the benign and the malignant nature of a given lymphoid infiltrate. The descriptive term AtLP may be used as an interim label when morphologic and molecular features are not sufficient to render a positive diagnosis of lymphoma.

Proposed Etiologies of AtLP

In the author's opinion, AtLP are associated with some infections, immunologic conditions and the intake of certain drugs. A possible speculation is that a combination of some of these etiological factors may play a role in the development of AtLP. Infectious agents associated with the development of AtLP include: Epstein–Barr virus (EBV), human herpesvirus-8, cat-scratch agent (Bartonella henselae), HIV and Toxoplasma gondii. In the author's experience, EBV infection seems to be a frequent cause of AtLP. In this condition, the immune suppression allows the escape of EBV-infected B cells and thus results in a polymorphous lymphoid proliferation composed of small B cells, plasmacytoid cells and immunoblasts. In situ hybridization of EBV-encoded RNA sequences in lymph nodes usually demonstrates five- to ten-fold more EBV-positive cells in lymphoid tissues of HIV-infected patients compared with the lymphoid tissue in normal patients.[26]

The development of AtLP may also reflect an abnormal immune response to several inciting stimuli. The immunologic disorders associated with the development of AtLP include rheumatoid arthritis, Sjögren's syndrome and systemic lupus erythematosus.

Lymphoid proliferations (benign conditions, AtLP and lymphomas) may be noted in patients with primary or congenital immunodeficiencies (X-linked lymphoproliferative syndrome, ataxia–telangiectasia, common variable immunodeficiency, Wiskott–Aldrich syndrome and severe combined immunodeficiency)[27,28] and autoimmune lymphoproliferative disorders. Patients with primary immune disorders are more likely to develop benign T-cell expansions of CD4-/CD8- α/β T cells than T-cell malignancies. HIV-positive patients may develop a spectrum of lymphoid proliferation including: reactive hyperplasia, AtLP, lesions resembling polymorphic B-cell post-transplant lymphoproliferative disorders and malignant lymphomas.[29]

Immunohistochemical and molecular analysis of the intraepithelial B cells occurring in Hashimoto's disease, Sjögren's syndrome and Helicobacter pylori-associated gastritis revealed predominantly monocytoid B cells and marginal zone cells as the effector cells. Monoclonal rearrangements (T-cell receptor γ chains and immunoglobulin heavy chains) were noted.[30,31] The lymphoid proliferation in these conditions may be considered as the precursors of lymphyoma (prelymphoma). In addition, prelymphoma may be associated with some infections such as STLV-1 retrovirus[32–34] and may represent a stage of malignant lymphoma.

AtLP may be associated with the intake of certain drugs such as cabamazepine, dilantin, NSAIDs and immunosuppressive drugs such as methotrextate, antithymocyte globulin, azathioprine, corticosteroids, cyclosporine, tacrolimus and methotrexate. Similar infectious agents, immunologic conditions and drugs are known to be associated with the development of benign conditions (reactive hyperplasia and lymphadenitis) and malignant lymphomas.[35,36]

Proposed Histogenesis of AtLP

In the author's opinion, AtLP may share some (but not all) of the biological abnormalities characteristic for lymphomas and therefore they may represent equivocal (precursor) lesions that have accumulated some but not all the genetic changes required for malignant behavior. The lack of one or more of the critical genetic alterations would not allow AtLP to behave as fully developed lymphomas. It is possible that AtLP represent multiclonal B cells, T cells or NK-cell lymphoproliferative processes in which there is an existence of occult abnormal B-cell, T-cell or NK-cell clones. Subclones of these cells may acquire independent genetic changes to produce non-Hodgkin lymphomas and Hodgkin lymphoma. Another speculation is that the multiclonal lymphocytes may proliferate in a polyclonal fashion until one clone emerges, acquires malignant characteristics and then develops into frank lymphomas. The occult precursor clone is not usually evident in histological sections but can be detected at the molecular level. Different lymphomatous lesions originating in the background of AtLP thus arise from accumulated genetic changes in subclones of atypical lymphocytes. During this process, the transformed lymphocytes (atypical lymphocytes) must, in principle, progress through a number of molecular alterations before they can generate clinically detectable lymphomas. These alterations likely result from loss of tumor-suppressor genes, activation of oncogenes and genetic instability (microsatellite and chromosomal instability). During this process, the atypical lymphocytes of AtLP must acquire many characteristics such as growth autonomy. With the development of one or more of these changes, only those premalignant subclones of atypical lymphocytes able to overcome the opposing selection pressure will subsequently develop into malignant ones. Atypical lymphocytes that survive genetic damage likely undergo more mutations. Clonal expansion of these mutated cells may subsequently promote their progression into malignant lymphomas. These speculations could explain the observation that AtLP can be forerunners of lymphomas. The spontaneous regressions observed in some cases of AtLP may be due to the presence of a strong host's immune system that can control the growth of the atypical lymphocytes. These propositions are presented in Figure 5. Separation of atypical lymphoid proliferation from reactive hyperplasia and malignant lymphomas is shown in Figure 6. However, it should be noted that sometimes AtLP are not necessarily premalignant and may very well represent a fully benign situation mimicking malignancy.

Figure 5.

Histogenesis of atypical lymphoid proliferation. Lymphomagenesis seems to be a multistep process requiring the sequential accumulation of a series of genetic alterations that culminate in cellular transformation. During this process, the transformed lymphocytes (atypical lymphocytes) must, in principle, progress through a number of molecular alterations before they can generate a clinically detectable lymphoma. The atypical lymphocytes of atypical lymphoid proliferations must acquire many characteristics, such as growth autonomy. These complex processes require alterations of fundamental cell processes such as accumulating mutations, inactivation of tumor-suppresor genes and developing defects in housekeeping genes. With the development of one or more of these alterations, only those premalignant subclones of lymphocytes able to overcome the opposing selection pressure will subsequently develop into malignant ones. Those cells that survive such DNA damage are likely to undergo more mutations. Clonal expansion of these mutated cells may subsequently evolve into lymphomatous cells.

Figure 6.

Separation of atypical lymphoid proliferation from reactive hyperplasia and malignant lymphomas. Lymphoid lesions that have been adequately examined and subjected to a full range of ancillary studies with inconclusive findings that fall short of a specific lymphoma diagnosis are labeled as 'atypical lymphoid proliferations'. For these lesions, three strategies may be considered: consultation, rebiopsy and close follow-up.

Clinical Features of AtLP

In the author's experience, AtLP may present clinically as a single enlarged lymph node, and as localized or generalized lymphadenopathy. Presentations of AtLP concerning of bad outcome (progression to malignant lymphomas) are: persistent lymphadenopathy without spontaneous regression within 6 months to 1 year, significant lymphadenopathy – that is, lymph nodes >2 cm in size – supraclavicular location and generalized lymphadenopathy associated either with organomegally (hepatomegally and/or splenomegaly) or systemic symptoms. However, no single clinical characteristic could distinguish AtLP from benign lymphoid proliferations or lymphomas. Neither is any single criterion available to establish evolution to malignant lymphoma. Although AtLP are not necessarily premalignant and may represent mimickers of malignant conditions, sometimes AtLP may result in death of the patient. This outcome may result from progression either to lymphomas (HIV-associated AtLP) or by damage to the immune system.

Morphologic Features of AtLP

A significant number of patients present with localized or generalized lymphadenopathy and are therefore candidates for lymph node biopsy. In the author's opinion, in addition to the more common benign conditions (reactive hyperplasia and lymphadenitis) and lymphomas, some biopsies may show worrisome features. This may result in the histologic diagnosis of AtLP – that is, lymph nodes with distorted architecture and lymphoid cells with worrisome cytological features – but findings are not sufficient for the diagnosis of lymphomas. For these challenging cases, the separation of AtLP from lymphomas depends on morphologic and molecular analysis.[23–25] Common morphologic patterns of AtLP are: AtLP with follicular pattern, AtLP rich in Reed–Sternberg-like cells, AtLP composed predominantly of B cells and AtLP composed predominantly of T cells.

Separation of AtLP Rich in Reed–Sternberg-like Cells from Classic Hodgkin Lymphomas

Distinguishing AtLP rich in reactive immunoblasts (cells indistinguishable from Reed–Sternberg cells) from classic Hodgkin lymphomas may be difficult on a morphologic basis. However, separation can be made based on several useful hints. To render the diagnosis of Hodgkin lymphoma, Reed–Sternberg cells should be found in a cellular milieu appropriate for one of the specific subtypes of Hodgkin lymphoma. Classic Reed–Sternberg cells are binucleate or multilobated cells with abundant cytoplasm, large eosinophilic nucleoli which can be round or linear surrounded by a clear halo. Some Hodgkin cells are large mononucleated cells with basophilic cytoplasm and big nucleoli. In contrast, immunoblasts of AtLP are not as large as mononuclear Reed–Sternberg cells, their nucleoli tend to be basophilic rather than eosinophilic and their cytoplasm is basophilic/amphophilic rather than eosinophilic.[37] Reed–Sternberg cells have a tendency for clustering versus even distribution of the reactive immunoblasts in AtLP. Reed–Sternberg cells usually have a characteristic immunophenotype (positive for CD15 and CD30 and negative for CD45/45RB).[38] By contrast, immunoblasts (CD45/45RB+) of AtLP tend to react with B-cell or T-cell markers but not CD15. CD20 stains a subset of Reed–Sternberg cells in some cases of classic Hodgkin lymphoma. CD3 is almost always negative in Reed–Sternberg cells. PAX-5 stains the majority of Reed–Sternberg cells. Fascin antibody also usually stains Reed–Sternberg cells in paraffin sections. EBV latent-membrane protein usually stains the EBV-positive cases of classic Hodgkin lymphoma, particularly mixed-cellularity Hodgkin lymphoma.[39,40] Lymphoid and histiocytic (L&H) cells of nodular lymphocyte-predominant Hodgkin lymphomas usually stain for CD20 and CD45/45RB and not for CD15 or CD30. L&H cells also stain for epithelial membrane antigen and CD79a; results of both are negative in Reed–Sternberg cells. A characteristic feature of nodular L&H is the presence of a wreath-like configuration of the CD57+ small lymphocytes around L&H cells. In contrast with Reed–Sternberg cells, fascin, CD74 and CD138 do not stain L&H cells. The typical background of classic Hodgkin lymphoma includes only scattered plasma cells.[41,42] The background of AtLP may be rich in activated lymphoid cells (CD30+ cells), which rarely occurs in abundance in Hodgkin lymphoma.[43,44] Thus, CD30 staining should be interpreted in the context of histological findings and reactivity to other markers for Reed–Sternberg cells.[45,46] By contrast, numerous lymphoplasmacytoid cells may be seen in AtLP. Lesions of interfollicular Hodgkin lymphoma may be misdiagnosed as lymphadenitis. Aspects indicating the malignant diagnosis are: displacement of the residual germinal centers and mantle zones by an infiltrate of Hodgkin cells in the interfollicular areas.[47] By contrast, lymphadenitis is characterized by interfollicular proliferation of lymphocytes, histiocytes, plasma cells and immunoblasts.[48,49] A case of AtLP with large cells resembling Reed–Sternberg cells, but insufficient for definitive diagnosis of Hodgkin lymphoma is shown in Figures 7 & 8. The most important aspects concerning the distinction between AtLP rich in immunoblasts (Reed–Sternberg-like cells) and Hodgkin lymphoma are summarized in Table 2 .

Figure 7.

Atypical follicular proliferation with large cells resembling Reed–Sternberg cells, but insufficient for definitive diagnosis of Hodgkin lymphoma. A 61-year-old male presented with an enlarged right inguinal lymph node. Grossly, the specimen consisted of a single node measuring 4.0 × 1.7 × 1.3 cm. Sections revealed a lymph node with an altered architecture with a suggestion of nodularity. At low power, the node has a mottled appearance created by loose clusters of epithelioid cell granulomas admixed with small lymphocytes that are not cytologically abnormal. Also present in the background are modest numbers of eosinophils and scattered larger more atypical cells with prominent nucleoli and bubbly chromatin. A few mummified cells are also present. Cells with the morphologic features of Hodgkin cells are not plentiful but can be found and there are even rare binucleate cells, which suggests Reed–Sternberg cells (A–F). The immunohistochemical stains were performed. The CD20 stains show relatively few B cells, which morphologically appear to be small lymphocytes (G). CD20 stain appears to be negative in the large cell population. Most of the background cells are positive for (H) CD3, (I) CD2, (J) CD5, (K) CD7 and (L) CD8. The large atypical cells are negative for these stains. (M) CD15 and (N) CD30 are positive in few atypical cells with both membrane and Golgi staining. Pax 5 stain shows scattered clusters of small lymphocytes marking positively as well as interspersed larger atypical cells with macronucleoli in the regions denominated by histiocytes and T cells (O–P). A single predominant PCR amplicon was generated in the assay for clonal T-cell receptor-γ arrangements, indicating the presence of a clonal population of T lymphocytes. However, the finding of an apparent clonal band with T-cell receptor-γ is felt to lack specificity in the absence of α or β rearrangements. The morphologic features are those of atypical lymphoproliferative disorder, suggestive of lymphocyte-rich Hodgkin's lymphoma but insufficient for definitive diagnosis. A second biopsy to attempt proper analysis of the lesional tissue was suggested.

Separation of AtLP With Follicular Pattern From Follicular Lymphomas

The distinction between follicular hyperplasia and follicular lymphoma may be straightforward. A more challenging distinction is faced in AtLP with follicular pattern with the lymph node architecture being partially or completely effaced by lymphoid follicles. The germinal centers of some follicles may be poorly demarcated and lack well-formed lymphoid cuffs. Clues favoring follicular lymphoma are: back-to-back follicles disposed throughout the entire nodal parenchyma with little interfollicular tissue,[50,51] predominance of centrocytes (small cleaved cells) in the follicles, dysplastic follicular center cells and presence of atypical cells in the interfollicular regions. Other diagnostic aids favoring lymphoma include: lack of tingible body macrophages, absence of incomplete mantle, lack of polarization in the follicles and presence of follicles in the perinodal tissues.[50,51] Separation of AtLP with follicular pattern from follicular lymphoma is shown in Table 3 .

Several immunohistochemical findings favor the diagnosis of follicular lymphoma over AtLP with follicular pattern. Positive staining of follicles center cells for BCL2 protein is seen in 85% of the lymphomas and therefore represents a useful tool.[23,52] However, BCL2 staining is not specific for the diagnosis of follicular lymphoma and can be seen in B-cell lymphomas and unstimulated follicles.[53,54] In follicular lymphoma, the interfollicular zone is densely backed by CD20+ or CD79+ B cells, indicative of invasion of interfollicular zone by lymphoma. In AtLP, the interfollicular zone is usually rich in T cells with few isolated B cells.[50,51] Normally, few or no CD10+ cells are seen in this region.[55,56] A significant number of CD10+ cells in the interfollicular zone favor the diagnosis of follicular lymphoma. Light-chain restriction in follicle-center cells is seen only in follicular lymphoma. In AtLP, there is polytypic staining for immunoglobulin light chain, often in a network-like pattern rather than discrete membrane or cytoplasmic staining of follicular center cells.[57–60] Other helpful features that help separate AtLP with a predominant follicular pattern from follicular lymphoma include a low Ki-67 (proliferation) and aberrant MT2 expression in lymphomas.[61] Molecular analysis shows t(14;18)(q32;q21) chromosome abnormality in about 85% of the follicular lymphomsa. This translocation involves the immunoglobulin heavy-chain gene locus on chromosome 14 and the BCL2 gene locus on chromosome 18. Methods used for molecular analysis include cytogenetics, PCR or FISH. Separation of AtLP with follicular pattern and follicular lymphoma is shown in Table 3 . Cases of AtLP with predominant follicular patterns are shown in Figures 8 & 9. The most important aspects concerning the distinction between AtLP with follicular pattern and follicular lymphoma are summarized in Table 3 . A case of follicular lymphoma versus AtLP with follicular pattern is shown in Figure 9.

Figure 8.

Atypical lymphoproliferative disorder, suggestive of lymphocyte-rich Hodgkin lymphoma but with insufficient features for definitive diagnosis. A 70-year-old female patient presented with a single enlarged groin lymph node. The specimen consisted of a lymph node measuring 1.8 cm in diameter with soft consistency and grayish cut section. Histologic sections show an enlarged lymph node with a thick capsule and vaguely nodular parenchyma with a heterogeneous cell population. Although there are follicles with reactive germinal centers present, there is partial effacement of the parenchyma with a mixed cell population including a significant number of granulocytes, some cells with histiocytic morphology and a mixture of small, intermediate size and large lymphocytes and occasional plasma cells. There is no necrosis noted. In the areas where the granulocytes are more densely populated there are also significant number of mononuclear large cells with vesicular nuclei and prominent nucleoli, some of these large cells show significant indentation (lobulation) of the nuclear membrane and rare bi-/multi-nucleation. Some of these cells have the features of Reed–Sternberg cells, but they lack the cherry-red, inclusion-like nucleoli of the classic Reed–Sternberg cells. Although there are no well-developed necrotic foci in this lymph node, there are occasional cells with apoptotic bodies and there are large cells with mummified morphology (A–F). The pertinent immunohistochemical findings are interesting. CD20: B cells (including a significant number of large cells) are positive (G & H); CD3: B cells (including significant number of large cells) are positive (I & J); BCL2: patchy staining (K & L); CD30: occasional large cells are positive (M & N); and CD15: granulocytes and rare mononuclear cells are positive (no convincing evidence of positive reaction observed with the large cells, Reed–Sternberg-like cells) (O & P). CD5: T cells (including a significant number of large cells) are positive (not shown in the figure).

Figure 9.

Atypical follicular proliferation with predominant follicular pattern, favor follicular lymphoma, grade 1. A 79-year-old male patient presented with a history of biliary colic. Cholecystectomy was performed. The gallbladder was grossly unremarkable and measured 10 × 3.5 × 3 cm. The serosal surface was dull and grayish-tan in color. The lumen contains thick, pale green bile. No calculi were noted. The gallbladder wall measured 0.2 cm in thickness and the mucosal surface was velvety green. Histologically, the gallbladder was unremarkable. Two lymph nodes were noted adjacent to the cystic duct measuring 0.9 × 0.7 × 0.2 cm and 1.5 × 0.6 × 0.5 cm. (A–D) Sections from the lymph nodes show effacement of the nodal architecture by closely apposed neoplastic follicles with a back-to-back appearance. These follicles are uniform size and shape. Polarity, mantle zones and tingible body macrophages are absent in these neoplastic follicles. The neoplastic cells in follicles express CD20 (E–H), BCL-2 (I–L) and CD10 (M & N). Ki-67 shows a lower mitotic rate (O & P). Six months after the surgery, the patient developed splenic focal lesions and inguinal lymphadenopathy.

Separation of AtLP Composed Predominantly of B Cells From B-cell Lymphomas

AtLP composed predominantly of B cells can be separated from non-Hodgkin lymphomas by morphologic analysis of several aspects. The following features favor AtLP: distorted nodal architecture but with intact sinuses and/or recognisable paracortex (obliterated in lymphoma) and persistent reactive foci with the classic features of underlying etiology.[62] In AtLP, the lymphoid cells usually do not form expansible tumour masses or erode the mantels of subjacent reactive follicles (expansile masses in lymphoma). Angiocentric and angiodestructive growths are totally absent in AtLP. By contrast, these features may be seen in lymphomas.[63,64] The lymphoid infiltrate of AtLP usually lacks significant collections of lymphoid cells with strikingly folded angulated nuclei, abnormal granular chromatin or clear cytoplasm (striking dysplastic lymphoid cells in lymphoma).

Immunohistochemical studies play an important role in the separation of AtLP composed predominantly of B cells from B-cell lymphomas.[11] AtLP composed predominantly of B cells usually lack evidence of abnormal immunoarchitecture or aberrant immunophenotype. Abnormal immunoarchitecture is defined as the presence of extensive collections of B cells (sheets of CD20+ or CD79+ cells, at least half of a low-power field using ×4 objective) with rare interspersed T cells (CD3+). Abnormal immunoarchitecture favors B-cell lymphomas. In AtLP, the cells expressing κ light-chain outnumber cells expressing λ-light chain in a 2:1 ratio. In B-cell lymphomas, cells usually express either κ or λ light chain (light-chain restriction). The presence of clear-cut Ig (κ: λ ratio more than >10:1 or >1:10) is diagnostic of lymphomas. This restriction is a good clue supportive of malignant diagnosis.[57]

Aberrant immunophenotype is defined as absence of antigens normally expressed on the B cells. Aberrant immunophenotype favors the diagnosis of lymphoma and argues against AtLP. For instance, approximately 25% of the B-cell lymphomas lack the expression of immunoglobulin. Intermediate- and high-grade B-cell lymphomas are most often immunoglobulin negative.[57] Lymphocyte function-associated antigen-1 may be absent in 50% of the high-grade lymphoma and 30% of the intermediate- and low-grade lymphomas.[65] Also, in aberrant immunophenotype, B-cell lymphomas (but not AtLP) coexpress markers not expressed by B cells (CD43, CD5 and cyclin D1).[4,66–69] Addition of CD43 to the CD3/CD20 combination adds great value to the diagnostic combination. CD43 is expressed on normal T cells and macrophages/histiocytes. It is not typically coexpressed on normal B cells, while is often expressed by malignant B cells, including several subtypes of small B-cell lymphoma.[4,66,67] All normal and reactive lymphoid cells are cyclin D1 negative. Also, expression of cyclin D1 by a lymphoproliferative lesion favors the diagnosis of lymphoma and not AtLP.[15,67,68,70] The most important aspects concerning the distinction between AtLP composed predominantly of B cells and non-Hodgkin lymphomas are summarized in Table 4 .

Separation of AtLP Composed Predominantly of T Cells From T-cell Lymphomas

AtLP composed predominantly of T cells usually show paracortical expansion and may be misdiagnosed as peripheral T-cell lymphoma. Clues favoring AtLP are: the paracortical expansion has a mottled appearance at the scanning magnification with the presence of some hyperplastic follicles, and the infiltrate is composed of a mixture of discrete cell types (small lymphocytes, immunoblasts, histiocytes and granulocytes). By contrast, clues favoring peripheral T-cell lymphoma are: complete effacement of the nodal architecture with the presence of atrophic intervening follicles, and the infiltrate is composed of a diffuse proliferation of atypical lymphoid cells (CD3+ or CD2+). The cells demonstrate an aberrant T-cell immunophenotype and clonal T-cell receptor gene rearrangements. As AtLP may be composed predominantly of T-cell population, the presence of diffuse CD3+ cell infiltrate of T cells is not sufficient for diagnosis of T-cell lymphomas. Also, other pan-T-cell markers (CD2, CD5 and CD7) are usually expressed by AtLP. However, deficiency (less than 50% loss) or complete loss of expression of pan-T-cell markers (CD2, CD5 and CD7) favors the diagnosis of lymphoma rather than AtLP.[71–74] CD7 deficiency can be seen in benign cutaneous lymphoid infiltrate and therefore this criterion in isolation is not discriminatory. However, when used together with other immunophenotypic (loss of other pan T-cell markers, discordance of dermal and epidermal expression of CD5) and molecular (such as T-cell receptor gene rearrangements) features, CD7 deficiency or loss can be useful.[71,73,74] Also, double-positive phenotype CD4+/CD8+ or double-negative CD4/CD8-phenotype favors the diagnosis of T-cell lymphomas.[71,73,74] However, it should be noted that the double-negative CD4/CD8 phenotype may be seen in immunologic disorders such as autoimmune lymphoproliferative syndrome, which had FAS or caspase mutations.[75,76] The most important aspects concerning the distinction between AtLP composed predominantly of T cells and peripheral T-cell lymphoma are summarized in Table 5 .

AtLP with prominent mixed inflammatory cell infiltrate and some atypical cells may be present in the nasal cavity mucosa and could be misdiagnosed as nasal cavity NK/T-cell lymphomas. The morphologic features that support the diagnosis of NK/T-cell lymphoma are: admixture of inflammatory cells, prominent necrosis, vascular invasion and atypical lymphocytes (varying from small- and medium-sized cells to large cells). The latter are positive for CD2, CD43 and NK-cell antigen CD56; negative for CD3, CD5, CD57 and CD11. Large groups of Epstein–Barr virus-encoded RNA-positive cells and high proliferative fraction in the atypical cells (Ki-67 more than 80%) are seen in NK/T-cell lymphomas.[77,78] Clues favoring AtLP are the absence of EBV-positive cells and possible detection of some etiological agents (microorganisms).