In this study, we present novel evidence for an association between chronic pulmonary T. whipplei infection and PMZL. Pulmonary T. whipplei infection may promote neoplastic transformation of B cells, or neoplastic B cell proliferations may provide a niche for T. whipplei persistence in the lung. In fact, individual cases have been described in the literature that suggest a pathophysiological connection between T. whipplei and B-cell lymphomas, in line with our hypothesis[18–20, 27, 28].
The multifaceted nature of chronic Whipple’s disease is likely due to a combination of factors, both host- and pathogen-dependent, which ultimately result in a failure to clear the bacterium, leading to chronic intracellular oxidative stress and failed antigen presentation. Host-dependent factors include an increased likelihood to develop chronic Whipple’s disease or persistence in patients with HLA-B27 positivity[29], suggesting an impairment of cell-mediated immunity[17]. Macrophage dysfunction has been postulated to be a hallmark of chronic Whipple’s disease, for example due to a decrease of interleukin-12 (IL-12), disturbed phagocytosis and antigen presentation. This, in turn, decreases interferon gamma (IFNγ), dysregulates NK-, T- and B-cell response[14, 30, 31], and ultimately contributes to lymphomagenesis[10].
Another important factor in the pathogenesis of chronic Whipple’s disease is NF-κB-mediated signalling, downstream of bacterial sensors such as Toll-like receptor 4 (TLR4)[32]. Previous studies suggest that bacterial infections may have activating effects on the B-cell receptor (BCR) and the NF-κB pathway in B cells, further supporting the pathophysiological role of chronic antigen stimulation in lymphomagenesis[33]. It has been shown that NF-κB-mediated oxidative stress can induce de novo genomic rearrangements, as was previously shown for the TMPRSS2-ERG gene fusion in prostate cancer[34]. The role of NF-κB in PMZL lymphomagenesis is also supported by HTS analyses[35], which identified mutations and translocations of TNFAIP3, BCL10, CARD11, MYD88, TNFRSF14 and MALT1 in almost 60% of PMZL. All of these alterations have been shown to influence the activity of the NF-kB pathway[36, 37]. Interestingly, both cases of T. whipplei-associated PMZL showed comparatively few mutations and, in particular, no mutations in genes related to the NF-κB or the Notch signalling pathway and no mutations in chromatin modifier encoding genes characteristic for PMZL[25, 35]. A similar pattern is observed in diffuse large B-cell lymphoma (DLBCL), where the mutational profiles and load differ significantly between EBV-positive and negative cases[38, 39]. These findings suggest that T. whipplei infection may compensate for the missing mutations in these PMZL, thus promoting lymphomagenesis.
Our data on the association of T. whipplei and PMZL supports a potential causative link between T. whipplei infection and PMZL. MZL in other localizations are similarly associated with various chronic infections[5–7], and patients with PMZL have been successfully treated with prolonged clarithromycin therapy, further corroborating a potential infectious cause of this B-cell lymphoma[40]. However, several uncertainties remain. Patient 2 had a history of M. tuberculosis, which might have contributed to the development of PMZL through chronic inflammatory stimulation. Only a minority of pathogens are known to chronically persist in the lung, which is a fundamental prerequisite for chronic inflammatory stimulation. Indeed, M. tuberculosis DNA has been found by in-situ PCR within lungs of individuals who have died from causes other than tuberculosis[41, 42]. A previously reported case suggested a temporal link between the occurrence of PMZL and pulmonary mycobacterial infection and regression of PMZL after antibiotic treatment for concomitant pulmonary tuberculosis[43]. Based on this data, we cannot exclude a pathophysiological connection between chronic mycobacterial infection and the occurrence of PMZL in patient 2. However, a PCR on the lung tissue taken at the time of PMZL diagnosis was negative for mycobacteria (M. tuberculosis and atypical mycobacteria).
A history of perioperative broad-spectrum antibiotic treatment in 4 of 22 cases might have influenced the negative PCR results of the cohort, although the rate of tissue penetration of differing antibiotic regimens should not be overestimated. In general, antibiotic intake can cause false negative results in PCR analyses, as demonstrated in the second lung resection specimen of the index patient, which turned out negative T. whipplei after four months of antibiotic treatment (Fig. 1). The elimination of causative bacterial species by means of antibiotic therapy could eventually become a component of PMZL treatment, similar to Helicobacter eradication in gastric MZL [44–47]. As other bacterial infections could be potential PMZL drivers, it will be important to study larger PMZL cohorts with a non-targeted approach such as metagenomics WGS, rather than by means of species-specific PCR only.
Non-targeted analyses conducted so far have shown no evidence of any causative link between any pathogen and PMZL and, in particular, have refuted the suggested role of A. xylosoxidans [10]. Our data supports this, suggesting that A. xylosoxidans is part of the normal lung microbiome and unlikely to play a role in the pathogenesis of PMZL.