Antigens play a critical role in the development of chronic lymphocytic leukemia (CLL) by binding to and stimulating leukemic precursor cells at some point during CLL ontogeny. Nevertheless, much remains unknown and further studies are necessary before an accurate model of antigen-drive can be ascertained. In this context, intraclonal diversification (ID) analysis of immunoglobulin (IG) genes could shed light on whether antigen involvement is restricted to the malignant transformation phase or if the triggering antigen(s) continuously stimulates the CLL clone. Hence, in Paper I we conducted a large-scale analysis of 71 CLL cases and revealed that 28/71 cases carried intraclonally diversified IGHV-IGHD-IGHJ genes. Although most cases showed no or low levels of ID, intense ID was evident within all subset #4 (IGHV4-34/IGKV2-30) cases. Subsequent analysis, in Paper II, of the clonotypic light chains revealed that the outstanding exception again related to subset #4. In such cases, the expressed IGKV2-30 gene was affected by targeted ID, analogous to their partner IGHV4-34 gene. Whilst these results convincingly argued for the role of antigen(s) in the development and evolution of CLL subset #4, this analysis was limited to depicting what was occurring at a single time-point and could not provide insight into the temporal dynamics of the CLL clones. Thus, in Paper III we conducted a longitudinal study of 8 subset #4 cases which enabled us to establish a hierarchical pattern of subclonal evolution. The observed ‘stepwise’ accumulation of mutations strongly supports a role for antigen selection in the pathogenesis of CLL subset #4. In Paper IV we reported a subset of IgG-switched CLL patients with coexisting trisomies of 12 and 19, and propose that the emergence of trisomy 18 in such cases represents a clonal evolution event suggestive of selection due to a clonal advantage. Paper V focused on the IGHV3-21 gene, an adverse prognostic factor in CLL. Since ~60% of IGHV3-21-expressing cases carry stereotyped B cell receptors, recognition of a common antigenic epitope, perhaps of pathogenic significance, is envisaged. Therefore, we investigated IGHV3-21 gene frequency within a Swedish population-based cohort and assessed the impact of stereotypy on clinical outcome. Taken collectively, this thesis provides molecular and genetic evidence for the role of antigen in CLL pathogenesis by convincingly demonstrating that clonal evolution, at least for certain subsets of CLL, is functionally driven rather than a consequence of clonal expansion promoted by nonspecific stimuli.

Advancements in prognostication have improved the subdivision of chronic lymphocytic leukaemia (CLL) into diverse prognostic subgroups. In CLL, IGHV unmutated and IGHV3-21 genes are associated with a poor-prognosis, conversely, IGHV mutated genes with a favourable outcome. The finding of multiple CLL subsets expressing ‘stereotyped’ B-cell receptors (BCRs) has suggested a role for antigen(s) in leukemogenesis. Patients belonging to certain stereotyped subsets share clinical and biological characteristics, yet limited knowledge exists regarding the genetic and epigenetic events that may influence their clinical behaviour. This thesis aimed to, further investigate Swedish IGHV3-21-utilising patients, screen for genetic and DNA-methylation events in CLL subgroups/subsets and study DNA methylation over time and within different CLL compartments. In paper I, IGHV gene sequencing of 337 CLL patients from a Swedish population-based cohort revealed a lower (6.5%) IGHV3-21 frequency relative to previous Swedish hospital-based studies (10.1-12.7%). Interestingly, this frequency remained higher compared to other Western CLL (2.6-4.1%) hospital-based cohorts. Furthermore, we confirmed the poor-outcome for IGHV3-21 patients to be independent of mutational and stereotypy status. In paper II, genomic events in stereotyped IGHV3-21-subset #2, IGHV4-34-subset #4 and subset #16 and their non-stereotyped counterparts were investigated via SNP arrays (n=101). Subset #2 and non-subset #2 carried a higher frequency of events compared to subset #4. A high frequency of del(11q) was evident in IGHV3-21 patients particularly subset #2 cases, which may partially explain their poor-prognosis. In contrast, the lower prevalence of aberrations and absence of poor-prognostic alterations may reflect the inherent low-proliferative disease seen in subset #4 cases. In papers III and IV, differential methylation profiles in IGHV mutated and IGHV unmutated patients were identified using DNA-methylation microarrays. CLL prognostic genes (CLLU1, LPL), tumor-suppressor genes (TSGs) (ABI3, WISP3) and genes belonging to TGF-ß and NF-kB/TNFR1 pathways were differentially methylated between the subgroups. Additionally, the re-expression of methylated TSGs by use of methyl and deacetyl inhibitors was demonstrated. Interestingly, analysis of patient-paired diagnostic/follow-up samples and patient-matched lymph node (LN) and peripheral blood (PB) cases revealed global DNA methylation to be relatively stable over time and remarkably similar within the different compartments. Altogether, this thesis provides insight into the aberrant genomic and DNA methylation events in divergent CLL subgroups. Moreover this thesis helps distinguish the extent to which DNA methylation changes with respect to time and microenvironment in CLL.

Les Lymphomes Primitifs Vitréo-Rétiniens (LPVR) représentent un sous-type de Lymphome Primitif du Système Nerveux Central (LPSNC). Ces hémopathies très rares sont caractérisées par leur localisation anatomique atypique, dans des sites physiologiquement dépourvus de lymphocytes B. Les lymphomes du SNC sont rattachés histologiquement aux Lymphomes B Diffus à Grandes Cellules (LBDGC) de type post-germinatif (ABC). L’objectif de notre étude était de définir le répertoire immunologique (chaînes lourdes et légères) des LPVR et des LPSNC, et de les comparer aux LBDGC. Nous avons mené une étude immunologique détaillée de ces tumeurs afin de rechercher des éléments de réponse expliquant ces localisations ectopiques. Notre projet, réalisé sur la plus grande série de LPVR à ce jour, a mis en évidence un biais de répertoire majeur, avec une sur-représentation massive du gène IGHV4-34 (63,6% des cas), significativement plus utilisé dans les LPVR comparativement aux LPSNC et aux LBDGC systémiques. Bien que la proportion de ce gène soit élevée dans d’autres SLP, cette fréquence n’a jamais été atteinte. Un subset a été décrit pour 50% des LPVR utilisant le gène IGHV3-7. Ces données suggèrent fortement l’implication d’un antigène dans leur développement. En conclusion, le LPVR représente un modèle surprenant et singulier de lymphome dirigé par l’antigène, dont l’identification offrirait des perspectives physiopathologiques et thérapeutiques prometteuses. / Primary vitroretinal lymphoma (PVRL) is a high-grade lymphoma considered as a subtype of primary central nervous system lymphoma (PCNSL). Unusual localization is a feature of these rare entities. The vast majority of cases are diffuse large B cell lymphoma (DLBCL), mostly of activated B-cell (ABC). To investigate whether PVRLs display a specific IG repertoire contributing to explain their unusual localization, we analysed in detail the IG heavy and light chain sequences from PVRL and PCNSL cases, and we compared their repertoire to that of a publicly available IG heavy chain sequences dataset from systemic ABC-type DLBCLs. Our study was carried out on the largest PVRL series reported to date and showed that PVRL displayed a strikingly biased repertoire as the IGHV4-34 gene was used in 63.6% of cases. The frequency was significantly higher in PVRL compared to PCNSL and DLBCL. This gene has been repeatedly found to be preferentially used in various B-cell malignancies, but never to such an extent. Half of PVRL cases expressing the IGHV3-7 gene had stereotyped VH CDR3 features (subset). Altogether our data showed that PVRLs display a very biased IG repertoire strongly suggesting that antigen selection plays a major role in their development. Thus, PVRL display a highly restricted IG repertoire indicative of antigen selection, and distinct from that of PCNSL. Antigen(s) identification may provide promising perspectives in physiopathology concepts and therapeutic approaches.