B-Cell Receptors of Primary Central Nervous System Lymphoma Recognize Antigens in the Brain

INTRODUCTION: The pathobiology mediating the development of primary central nervous system lymphoma (PCNSL) and its exclusive manifestation in the brain and spinal cord is still poorly understood. The monoclonal B-cell receptor expressed in PCNSL cells have a germinal center typology, suggesting previous antigen contact and antigen-based selection during B-cell development. Furthermore, sequence analysis of B-cell receptors in various patients revealed a bias in the immunoglobulin repertoire in PCNSL, potentially indicating that distinct epitopes might elicit such an immune response. Nevertheless, the nature of potential antigens containing such epitopes is unknown. Here, we provide the first evidence that such antigens may be located within the central nervous system itself, which might contribute to the specific organ tropism of PCNSL.

METHODS: Cryopreserved tissue of five randomly chosen PCNSL cases and three control cases with mantle cell lymphoma were used for this study. The variable heavy and light chains of the B-cell receptors were amplified from extracted RNA and cloned in IgG1 expression vectors. Recombinant B-cell receptors were expressed and purified as soluble human immunoglobulins. To explore the potential recognition of central nervous system antigens, murine cryo- and paraffin-embedded brain tissue sections were immunostained using these PCNSL or mantle cell lymphoma-derived immunoglobulins as primary antibodies.

RESULTS: Four out of five recombinant B-cell receptors from PCNSL but none of the MCL-derived control immunoglobulins recognized distinct anatomical structures in the brain which resulted in unambiguous immunostaining especially in regions with high neuronal density. The staining of brain tissue was associated with neuronal cells in all four positive cases. Staining was stronger in the cerebellum and the hippocampus compared to other areas of the brain and staining intensity varied broadly among the four positive cases. In the cerebellum, all four positive B-cell receptors recognized the cytoplasm of Purkinje cells, the largest neurons in the brain (therefore, staining could be readily identified). Two of the B-cell receptors also recognized the Purkinje cell’s dendrites. Three of the PCNSL-derived immunoglobulins also stained the granule cells, the most frequent neuronal cell type in the brain. Two of those stained in the cytoplasm and one showed a patchy but strong staining pattern in the nucleus of these cells. All of these stainings including negative controls were reproduced multiple times in independent experiments.

CONCLUSION: B-cell receptors of primary central nervous system lymphomas recognize neuronal structures in the brain in an individually case-dependent pattern. This might point towards an important role of antigen recognition in the pathobiology and specific organ tropism of this lymphoma.