IM is a benign and self-limited lymphoproliferative disease, mainly occurs in adolescents and young adults, with most patients caused by EBV infection11, 12. While, approximately 10% of cases arise from infections by other viruses, such as cytomegalovirus, HIV, and hepatitis B13. The diagnosis of IM is usually made based on clinical presentations (triad of pharyngitis, fever, and lymphadenopathy) and serological examinations (positive anti-VCA IgG and IgM, or positive monospot test together with IgG or IgM)5, 11, 12. However, atypical clinical presentations occasionally occur, including the ages of patients over 30, generalized or isolated lymphadenopathy at unusual sites, negative heterophile antibody tests, absence of atypical lymphocytosis in the peripheral blood smear etc., in which biopsies are needed6-8. In this case, the patient is atypical, due to his elder age (31-year-old) and long disease course (3 months).
Pathological features of IM are nonspecific. At low magnification, shrink of the lymphoid follicles are usually found, with interfollicular regions significantly enlarged. At high magnification, characteristic findings include B cells of different differentiation stages, i.e., activated lymphoblastic cells, immunoblasts, plasma-like cells, and mature plasma cells, with different sizes and morphologies. The specific feature is the proliferation of immunoblasts, which are large in size and rich in clear cytoplasm, giving a mottled appearance at low magnification. Mitotic figures and nuclear fragments are commonly detected. Increased small blood vessels, as well as necrosis can be seen in a few cases. In a minority of cases, small sheets of monocyte-like B cells can be found6. Immunohistochemical profiles revealed that CD3+ T cells are the major small lymphocytes within the expanded interfollicular zone. Active lymphoid-like blasts and immunoblasts which are CD20 and CD30 positive are scatteredly distributed, and are partially positive for CD30 with variable intensity. In all EBV infectious cases, nuclear EBER positivity can be detected in large-, medium- and small-sized lymphocytes.
Actually, the pathogenesis of IM is a continuous spectrum following EBV infection, and can be divided into four stages, i.e., latent stage, early stage, middle stage, and late stage.
(1) Latent stage: as EB viruses invade the body, they first infect B cells, resulting in a massive proliferation of B cells and induction of humoral immune responses. To prevent the proliferation of B cells, cytotoxic T lymphocytes are activated. Thus, this stage presents as the proliferation of B lymphocytes in the lymphoid follicles and cytotoxic T lymphocytes in the interfollicular zone, with EBV-infected B lymphocytes less than 5%. Latent stage of IM is similar to toxoplasmic lymphadenitis. However, the features of toxoplasmic lymphadenitis not only include the presence of immunoblasts and plasma cells, but also include necrosis and Langerhans cells. PCR and serological tests can be helpful to distinguish the pathogens14.
(2) Early stage: this stage presents as the proliferation of the large-, medium-, and small-sized cells in the paracortex region, among which large B lymphocytes significantly proliferated (accounting for 5%~75%). At this stage, IM should be differentiated from diffuse large B-cell lymphoma, no otherwise specified (DLBCL, NOS) and EBV-positive DLBCL. IM can be distinguished from DLBCL, NOS by the intact structure of the lymph nodes, the reactive proliferation of mixed cell types, as well as the immunophenotypes (scattered CD20 positivity with different intensities in IM vs. diffusely strong CD20 positivity in DLBCL, NOS). EBV is generally negative in DLBCL6. In EBV-positive DLBCL, the positive signals of EBV can be exclusively detected in large neoplastic cells, distinguishing from IM which shows EBV positive signals in polymorphous lymphocytes2.
(3) Middle stage: this stage presents as the continuous proliferation of the large-, medium-, and small-sized lymphocytes, most of which are medium- to large-sized T lymphocytes, with CD8+ cytotoxic T lymphocytes accounting for 35%～50%. Meanwhile, the number of B lymphocytes is decreased. At this stage, IM should be differentiated from anaplastic large cell lymphoma (ALCL) and classic Hodgkin lymphoma (CHL). In ALCL, the proliferative cells are frequently adhesive to each other, with the nuclei exhibiting in horseshoe or kidney shapes, as well as the presence of characteristic Hallmark cells. ALCL also exhibits specific immunophenotypes, i.e., tumor cells are positive for CD30 and EMA; frequently positive for T lymphocyte-associated antigens and cytotoxic molecule (TIA-1, granzyme B, perforin, etc.); in some cases, tumor cells are positive for anaplastic lymphoma kinase (ALK). While, in IM, CD30 can be positively stained in some cases, and EMA is negative. The proliferative lymphocytes also show EBV latent membrane protein 1 (LMP1) positive15. Hodgkin lymphoma (HL) and IM are the most common EBV-associated diseases in western countries16. Although both IM and CHL have similar background of mixed cell types, eosinophils are rare in IM while usually numerous in CHL. Furthermore, the presence of Reed-Sternberg (RS)-like cells in IM can cause confusion with CHL. However, the RS-like cells in IM show OCT-2 and BOB.1 positive, but CD15 negative, whereas the RS cells in CHL show CD15 positive, with one of OCT-2 and BOB.1 lost17. In situ hybridization result shows that EBER can be detected in both large and small cells in IM, whereas EBER can only be detected in large cells in CHL, with much more positive cells in IM than in CHL.
(4) Late stage: Large T lymphocytes are continuously increased, with CD8+ T lymphocytes accounting for 50-75%, and EBV-positive B lymphocytes less than 5%. The proliferative T lymphocytes show basophilic cytoplasm, irregular nuclei, coarse chromatin, and more frequent mitotic figures. At this stage, IM should be differentiated from peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS). PTCL is more common in elder adults. The tumor cells are proliferative in a monoclonal manner, which show irregular cell morphology and light or transparent cytoplasm. The tumor cells in PTCL express antigens related to T lymphocytes, such as T cell receptor beta-chain (TCR beta). Clonal rearrangement of genes can be helpful in the differential diagnosis6, 18.
As IM progresses, the interaction between T and B lymphocytes leads to enhanced activity of suppressor T cells and macrophages, which block the proliferation of B lymphocytes. Ultimately, both B and T lymphocytes are decreased. Thus, IM presents as a self-limited disease. The treatment strategy for IM includes supportive treatment, rest, and analgesics. Since the pathological changes of IM at different stages mimic HL or non-HL, misdiagnosis of IM as lymphomas is common, resulting in inappropriate treatments. Thus, the pathologists should consider the differential diagnosis of IM and lymphomas at different stages. It is believed that in situ hybridization using EBER probes is helpful for differential diagnosis of IM2.