Lymphomas are characterized by the uncontrolled growth of clonal lymphocytes. Risk factors include age, family history, autoimmune disease, immunosuppression, exposure to chemicals or radiation, genetic risk factors, and specific bacterial or viral infections. While some viruses, such as EBV, HHV-8, and HTLV-1 can directly transform lymphocytes, the B cell dyscrasias and B cell lymphomas arising in patients chronically infected with HCV have long been suspected to be driven by chronic immune stimulation: Either indirectly by a chronic inflammatory environment such as in Helicobacter-driven lymphomas or more directly through chronic stimulation of the BCR pathway by HCV antigens3, 11–13. Yet, the exact pathophysiological phenomena underlying the association of HCV and lymphomas remained unclear.
In our work, we characterized the peripheral blood B cell architecture of patients with chronic HCV that – clinically – did not suffer from B cell dyscrasias such as cryoglobulinemia or lymphoma. We found a characteristic HCV fingerprint essentially consisting of lymphoma-like immunoglobulin gene usage, high somatic hypermutation, and stereotypic CDR3 motifs. Unexpectedly, this fingerprint persisted in the majority of patients even years after successful HCV therapy. Moreover, we pinpointed mutations of serine 31 within the CDR1 of the IGHV1-69 gene that had recently been identified to facilitate broad neutralization of antibodies against HCV E222. While serine 31 mutations have been reported in small cohorts of HCV-associated lymphomas (3/8 patients with S31T and 1/8 patients with S31N in27; 1/3 with S31T and 1/3 with S31N in28) we show that this residue is also frequently mutated in HCV-unrelated high-grade lymphomas.
It has been widely recognized over the past decade that the antigen receptor with its specific configuration is a powerful tumor promotor in lymphoma. Major revelations were the discovery of preferentially used immunoglobulin genes (e.g. IGHV4-34, IGHV4-59, or IGHV1-69) to the extent of virtually identical “stereotyped” receptor configurations13, 29–35, their reactivity with self-antigens29, 36–41, their autonomous signaling capacity through BCR-internal epitopes40, 42 as well as the clinical success of a wealth of new therapies interfering with antigen receptor signaling. Through these studies, it became clear that lymphoma BCRs harbor recurrent sequence motifs that are rare or even completely absent in healthy lymphocytes e.g. the chronic lymphocytic leukemia-specific light chain point mutation IGLV3-21R11043–48. The acquisition of enhancing IGHV1-69 “HCV-neutralizing” hotspot mutations during infection that are at the same time recurrent aberrations in BCRs of HCV-unrelated DLBCLs do add a whole new perspective on the potential risks underlying anti-viral immune responses. Our data suggest that in the case of HCV, broad virus neutralization may come at the cost of generating BCR variants with oncogenic potential that mimic the chronic active BCR signaling phenotype characteristic for lymphomas. In line with this, we observed that HCV-derived B cells expressing IGHV1-69-encoded BCRs with CDR1 hotspot mutations upregulated classical lymphoma drivers downstream of the BCR and MYC as well as NF-κB, MAPK, and NFAT pathway components such as CARD11, MALT1, RelB, PIM3, KRAS, MAPK8IP3, and NFATC1. As a consequence, the persistence of expanded B cell networks may not simply be regarded as an uncleared BCR-centered “HCV scar”, but as the result of imprinted lymphoma-like BCR signaling maintaining the persistence of these clonal networks. This idea is substantiated by our PHATE analysis that identified these gene expression profiles in patients with chronic HCV and those with SVR.
One mechanistic explanation of how upstream IGHV1-69 hotspot mutations allow engaging in lymphoma-like signaling comes from the superior responses to IgM ligation for one of the lymphoma-derived BCRs with mutated CDR1. It is plausible, that specific mutation-induced steric changes in receptors of this family with their inherent propensity for autoreactivity lower the activation threshold for cross-reaction with self-antigens. This is also illustrated by the light chain dependency of this activation effect. Interestingly, this BCR not only carries two CDR1 hotspot mutations but also shares other characteristics with classical elite-neutralizing HCV antibodies, e.g. the long and hydrophobic CDR3, the hydrophobic CDR2 and the rather high mutation rate of 94% identity to germline22. The requirement for multiple BCR alterations in combination with proper light chain usage might contribute to the only moderately elevated lymphoma risk of HCV patients.
From a clinical perspective, the substantial longevity of the HCV-induced oncogenic imprints despite cessation of antigenic pressure may provide an explanation for the increased lymphoma risk even after HCV cure that has been suggested by some cohort studies. One of these studies showed that three of 431 DAA-treated patients were afterward diagnosed with high-grade non-Hodgkin lymphoma; the prevalence in this cohort was 696 per 100,000 - about 30 times higher compared with the general population49. Also, El-Serag et al50 demonstrated no impact of DAA treatment on non-Hodgkin lymphoma risk. Our finding of lymphoma-like B cell programs in the absence of HCV antigenic pressure may explain why viral elimination induces remissions only in selected lymphoma cases.
Together, this data contributes a new perspective on how anti-viral immune responses may foster lymphomagenesis and should raise our awareness of elevated lymphoma risk in successfully treated HCV patients in the DAA era.