Since the beginning of the nineties, lymphoplasmacytic lymphoma (LPL) entity has been repeatedly challenged against marginal zone lymphomas, mainly splenic marginal zone lymphoma (SMZL). This is mainly due to difficulties in differential diagnosis related to overlapping morphological features. Various authors raise the question of the genuine existence of LPL, considering that plasma cell differentiation is a morphological aspect of marginal zone lymphomas. Moreover, some authors consider that secretion of an IgM monoclonal component at high levels, so-called Waldenström macroglobulinaemia (WM), is just a clinical/biological syndrome.1 Sequence analysis of immunoglobulin variable regions could be of interest in understanding the relationships between these two entities. For example, 1/3 of SMZL cases exhibit unmutated immunoglobulin variable heavy-chain (IgVH) genes, with a heterogeneous usage of VH segments.2 In contrast, IgVH somatic mutations are frequent with a predominance of VH3 usage in LPL patients.3 But, it is worthnoting that these results are not fully conclusive because they have been obtained on heterogeneous biological materials and in different separated series.
To clarify the relationship between these two entities, we analysed IgVH gene and somatic mutation pattern on homogeneous material consisting of infiltrated bone marrow trephine biopsies. Patients were retrospectively enrolled according to institutional regulations and after approval from the scientific committees of the tumour banks of the University Hospitals of Bordeaux. SMZL (n=11) and LPL (n=14) cases with bone marrow infiltration were retrospectively retrieved from our files.
Clinical and biological information of each patient are presented in Supplementary Table 1. The median age was 62 years (range, 43–78 years) and 67 years (range, 54–82 years) with a female-to-male ratio of 6/5 and 10/4 for SMZL and LPL patients respectively. Histopathological diagnoses were reviewed by two of us (MP and BP). For eight patients with SMZL, splenectomy allowed to confirm the diagnostic of SMZL. In the three cases remaining without splenectomy, diagnosis was proposed on immunocytological aspect and pattern of medullar involvement. Medullar infiltration of SMZL consisted of intrasinusoidal infiltration, isolated or associated with a nodular pattern. Lymphoma cells were small- to medium-sized, characterized by a pale cytoplasm surrounding a round or oval nucleus with dense chromatin and small nucleolus, without marked features of plasma cell maturation at both morphological and immunological levels. Tumour cells were CD20-positive (highlighting the intrasinusoidal infiltration), DBA44-positive in only four cases (36%) and CD5-, CD23- and CD138-negative. Labelling of CD138 evidenced plasma cell population (<5%) that was polytypic plasmocytosis (reactive or residual?) as shown by κ and λ light-chain labelling. All cases of LPL exhibited an interstitial, nodular or diffuse pattern. Lymphomatous infiltrate was composed of small lymphocytes with a variable degree of plasma cell maturation. Mastocytosis hyperplasia was frequent as assessed by Giemsa stain and CD117 labelling (n=14) (whereas present in only three cases of SMZL). Lymphomatous cells were CD20+, CD5− and CD23−. CD138 positivity was restricted to cells presenting marked plasmacytic differentiation and associated with a monotypic κ or λ light-chain expression. In contrast with SMZL cases, CD23 staining did never reveal follicular dendritic cell network. All LPL cases were DBA44-negative.
We performed the comparative analysis of IgVH gene sequences using Biomed24 primers and IMGT5 and Ig BLAST. The distribution of the VH, DH and JH segments among this series of patients is shown in Table 1, and was compared with published VH frequencies on normal B lymphocytes6, 7 (Supplementary Figure 1). VH usage was heterogeneous in SMZL cases with VH1 in 5/11 cases (45%), VH3 in 3/11 cases (27%) and VH4 in 3/11 cases (27%). JH4, JH6, JH2 and JH1 segments were found in 6/11 (55%), 3/11 (27%), 1/11 (9%) and 1/11 (9%) SMZL cases respectively. Eleven out of 14 cases (79%) of LPL had a rearranged VH gene belonging to the VH3 family, with an overrepresentation of the VH3–23 segment (6/14 cases, 43%). Ten out of 14 (71%) had used the JH4 gene, nine of them (64%) with both VH3 and JH4 rearranged segments. JH4-02 was the most commonly used individual JH4 allele. A slight overrepresentation of the DH gene D3-3 was observed in LPL (4/14 cases, 29%). Thus, VH and JH were different between SMZL and LPL (χ2 test, P=0.02). When compared to VH distribution of normal B cells, SMZL cases appeared to be characterized by an increased usage of VH1 (45% in our study versus 7.1% in normal B cells, P=0.01). As previously described in LPL,3 we found a marked VH3 restriction usage (78.3% of LPL cases in our study versus 33.1% in normal B cells) with a highly significant predominance of VH3-23 subtype (χ2 test, P<10−6).
Percentages of homology of VH segments with germline sequences are given in Table 1, and the percentage of somatic mutations of this series was given by 100 minus the percentage of homology (Figure 1a). Mean of somatic mutations was 5.7% (ranging 0–7%) in SMZL and 9.3% (ranging 5–16%) in LPL (Student's t-test, P=10−3). Five out of 11 patients (45%) with SMZL had a less than 5% mutation rate of IgVH gene, four of them with less than 2%. By contrast, all LPL cases had an IgVH mutation rate higher than 5% (Figure 1a). It is worthnoting that case M24 with an IgVH mutation rate of 11% was diagnosed as SMZL with an IgG secretory component at 18 g l−1.
The distribution of replacement (R) and silent (S) mutations in complementary determining (CDR) and framework regions (FR) could be statistically analysed according to the multinomial model proposed by Lossos et al.8 in 8/11 SMZL (those with at least three mutations in the CDR and/or FR sequences) and in all cases of LPL (Table 1). A significant increase of R mutations in CDR with a P-value <5% was observed in 0/8 SMZL and 3/14 (21%) LPL, and a significant decrease of R mutations in FR with a P-value <5% was seen in 3/8 (27%) SMZL and 10/14 (71%) LPL. This strongly suggests that somatic mutations of IgVH in LPL were submitted to a selection process eliminating those that could alter the structure of the immunoglobulin (decrease of R mutations in FR) and increasing the diversity of antigenic-binding sites (increase of R mutations in CDR), a result indicative of antigenic selection.9 Thus, LPL B cells do show the IgVH signature of germinal centre-experienced memory B cells with a high rate of IgVH mutations and a disequilibrium of the distribution of R mutations between FR and CDR.
Analysis of the diversity of VDJ junction evidenced a marked difference in the length of added nucleotides between SMZL and LPL. Translation of IgVH sequences clearly showed that CDR3 length was significantly higher in SMZL than in LPL (Figure 1b, non-parametric Mann and Whitney test, P=10−4). Regarding overlapping cases, cases L5 and L1 with intermediate CDR3 length were typical LPL cases with an IgM peak, and without spleen nor lymph node enlargement, and case M18 with short CDR3 was one of the three cases diagnosed as SMZL without splenectomy. Table 1 shows clearly that increase of the CDR3 length in SMZL corresponds to GC-rich N insertions, classically generated by Terminal deoxyribonucleotide Transferase.10 The enhanced length of CDR3 segment, the low mutation rate of IgVH genes and the homogeneous distribution of R mutations between FR and CDR suggest that tumour B cells of SMZL cases were selected by autoantigens outside germinal centres.11, 12, 13 It is worthnoting that digestive mucosa-associated lymphoid tissue lymphoma, believed to correspond to expansion of autoreactive B lymphocytes, also exhibits long CDR3 segment of IgVH genes.12
For the first time, we have extensively analysed IgVH gene sequences of bone marrow infiltration of both SMZL and LPL, with the major advantage of a homogeneous and comparable material between these two categories of tumour. We showed that low IgVH somatic mutation rate and long CDR3 segments but with no significant IgVH restriction usage are characteristics of SMZLs whereas IgVH of LPL cases frequently involved VH3 and JH4 and had a high mutation rate with an R mutation profile indicative of an antigen selection process. These results definitely demonstrate that the history to antigen exposure of these tumours diagnosed on the sole basis of clinical, morphological and immunohistochemical criteria, was completely different, arguing very strongly in favour of two different unrelated entities derived from B cells with two different fates and likely from two different compartments.
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This work was supported by the Cancéropôle Grand-Sud-Ouest, the Institut National contre le Cancer (INCa) contrat ACI-2004–2006 and ACI 2007–2008, Ligue contre le Cancer Comité du Limousin and Ligue Nationale contre le Cancer, Conseil Régional du Limousin. We thank the tumour bank of Hôpital Haut Leveque, Pessac, for providing biological samples.
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Parrens, M., Gachard, N., Petit, B. et al. Splenic marginal zone lymphomas and lymphoplasmacytic lymphomas originate from B-cell compartments with two different antigen-exposure histories. Leukemia 22, 1621–1624 (2008). https://doi.org/10.1038/leu.2008.24
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