Splenic marginal zone lymphoma (SMZL)

In the diagnosis of the various lymphoma entities, the cytomorphology and histology of blood, bone marrow, and lymph nodes are directional for guiding downstream diagnostics. On the one hand, the assessment of bone marrow smear provides an initial landmark statement as to whether lymphoma smear or lymphoma infiltration is present. Cytomorphology and histology are also helpful in assessing the degree of lymphocyte maturity.

Cytomorphologically, lymphoid cells with short, polar arranged villi, plasmocytoid lymphoid cells and inconspicuous lymphocytes can be detected in splenic marginal zone lymphoma in the peripheral blood smear. Morphologic differentiation from other lymphoma entities is usually not straightforward (Haferlach 2020).

Immunophenotyping allows clear determination of lineage affiliation to the T- or B-lineage in lymphomas. Furthermore, multiparametric flow cytometry is often indispensable for differentiating a reactive change from a lymphoid neoplasia, e.g. in EBV infections.

The neoplastic cells express the B-cell antigens CD19, CD20, CD22, CD79a/b, PAX5, CD27, and CD38 (dim). Expression of membrane-bound immunoglobulin (mostly IgM, less commonly IgD) and FMC7 is frequently present. In contrast, BCL6, annexin A1, CD103, cyclin D1, SOX11, and LEF1 are typically not expressed (WHO 2022). In general, CD5 and CD10 are also not expressed. However, in rare cases, there may be expression of CD5 that is not associated with an altered prognosis (Baseggio et al. 2010). Since CD5 is an important marker to differentiate splenic marginal zone lymphoma from other lymphoid entities such as chronic lymphocytic leukemia or mantle cell lymphoma, this must also be done based on morphological and genetic parameters. Previous studies also showed that the antigens CD11c, CD23, CD25, CD43 and cyclin D1 can be variably expressed. In contrast to mantle cell lymphoma, cyclin D1 is weakly expressed in splenic marginal zone lymphoma and may serve to differentiate (Arcaini et al. 2016, Piris et al. 2017).

Splenic marginal zone lymphoma is characterized by the absence of characteristic translocations found, for example, in follicular lymphoma (t(14;18); IGH::BCL2), mantle cell lymphoma (t(11;14); IGH::CCND1), and MALT lymphoma (t(11;18); BIRC3::MALT1, t(14;18); IGH::MALT1, or t(1;14); IGH::BCL10). For this reason, both chromosomal analysis and fluorescence in situ hybridization can significantly help to delineate the presence of phenotypically similar lymphoma entities. The presence of a 7q deletion may also indicate the presence of an SMZL.

The KLF2 (21%) and NOTCH2 (20%) genes are among the most frequently mutated genes in SMZL. However, they are found with low incidence or not at all in other CD5-negative lymphoma entities and thus may assist in differential diagnosis of SMZL. With decreasing incidence, mutations in TNFAIP3, KMT2D, TP53, as well as NOTCH1 are also found, but they are frequently found in other lymphoma entities. Where appropriate, however, detection may also be of diagnostic value depending on the context (Bonfiglio et al. 2022).

Overall, mutations of the NFkB pathway are very common in SMZL. In addition to the transcription factor KLF2, these include the following recurrent mutations: TNFAIP3 (13%), MYD88 (8%), TRAF3 (8%), CARD11 (5%), and BIRC3 (4%) (Clipson et al. 2015, Parry et al. 2015, Jaramillo Oquendo et al. 2019)

Because diagnosis of SMZL from bone marrow or peripheral blood is difficult, exclusion of genes typically mutated in other CD5-negative lymphomas may be helpful in this setting. This includes exclusion of BRAF V600E mutations (hairy cell leukemia), MYD88 L265P and CXCR4 mutations (lymphoplasmacytic lymphoma), and MAP2K1 mutations (splenic/s B-cell lymphoma/leukemia with prominent nucleoli (SBLPN); formerly hairy cell leukemia variant).