Self-Antigen Recognition by the B Cell Receptors of Follicular Lymphoma

Abstract 4124

There is accumulating evidence that FL is a non-autonomous disease. Firstly, the t(14:18) translocation, which places the bcl-2 proto-oncogene under control of the IgH promotor, is a cytogenetic hallmark of FL, yet it can be detected in B cells from healthy individuals, indicating that this translocation is not sufficient for disease. Phenotypically, follicular lymphoma tumor cells resemble antigen experienced germinal center B cells in that they have highly mutated B cell receptors (BCRs) and are continually undergoing somatic hypermutation (SHM). Despite the high risk of SHM leading to the introduction of nonsense mutations, follicular lymphoma cell continue to express functional BCRs on their surface. These features suggest a selective pressure for the maintenance of BCR expression and indicate the possibility that follicular lymphoma cells may be receiving survival signals through the BCR via antigen recognition. We hypothesize that follicular lymphoma BCRs can recognize self-antigens. To determine the frequency of tumors with self-reactive BCRs, recombinant tumor immunoglobulins were utilized in an indirect immunofluorescence assay with the HEp-2 human cell line. With this assay self-reactivity was detected in 43/99 (43%) of tested tumor immunoglobulins. Within the self-reactive tumor immunoglobulins there was a great diversity of staining patterns, indicating the absence of a unifying antigen that is recognized by all follicular lymphoma BCRs. Autoantigen protein microarrays were utilized to determine the frequency of tumor immunoglobulins reactive for known autoantibody targets associated with autoimmune diseases. Irrespective of HEp-2 reactivity status, none of the tumor immunoglobulins tested (0/50) were reactive against known auto-antigens. These observations indicate that the self-antigens recognized by tumor immunoglobulins might be categorically different from those recognized by the autoantibodies present in patients with autoimmune disease. In an effort to identify specific self-antigens being recognized we utilized the recombinant follicular lymphoma immunoglobulins to immunoprecipitate targets from HEp-2 cell lysates. For one patient's tumor immunoglobulin we identified myoferlin as a recognized self-antigen. The presence of ongoing SHM in follicular lymphoma tumors leads to the existence of multiple tumor clones. To assess how antigen recognition changed as this patient's tumor evolved through SHM we performed a rescue fusion, which immortalizes the tumor cells, halts SHM, and allows for the secretion of the tumor immunoglobulins. Recovered clones differed by a number of silent and replacement mutations, yet all remained HEp-2 reactive. Additionally, all clones maintained their ability to bind myoferlin, though with variable avidity. The observation that ongoing SHM did not break the self-reactivity of the patient's tumor supports the possibility that there is a selective pressure to preserve antigen recognition.