TFL Deletion Induces Incredible CXCL13 Secretion and Cachexia in Vavp-Bcl2Transgenic Mice

Diffuse large B-cell lymphomas (DLBCL) are heterogeneous diseases caused by several genetic aberrations. The novel post-transcriptional regulator gene called transformed follicular lymphoma (TFL) was first identified from t(2;6)(p12;q23), which appeared during the transformation of FL to DLBCL (Minagawa et al. Br J Haematol 2007). Normal human lymphocytes generally express TFL, but it is defective in some leukemia/lymphoma cell lines. TFL overexpression in such cell lines inhibited cell growth, suggesting that TFL functions as a tumor suppressor (Minagawa et al. Mol Cancer Res 2009). TFL locates in mRNA processing body in the cytoplasm and has the unique CCCH-type zinc finger motif functioning as RNase. TFL regulates several cytokines, including IL-2, IL-6, IL-10, TNF-α, and IL-17a, via mRNA degradation. In an experimental autoimmune encephalitis model, TFL null mice (TFL^-/-) demonstrated persistent paralysis, resulting from more infiltration of Th17 cells into CNS with markedly increased IL-17a mRNA levels. Therefore, a TFL-driven feedback mechanism for excessive inflammation is indispensable to suppress T-cell-mediated autoimmune diseases (Minagawa et al. J Immunol 2014).

TFL deletion examined by FISH using a 110kbp DNA probe containing an entireTFL locus was found in 12.8% of mature B-cell lymphomas (n=86, FL=30, DLBCL=40). However, the pathological significance of TFL deletion has not yet been clarified. To investigate how TFL loss affects lymphoma biology, we developed VavP-bcl2 transgenic (Bcl2-Tg)/TFL^-/-mice. Although the survival of TFL^-/- was comparable to the wild-type, Bcl2-Tg/TFL^-/- died about 19 weeks earlier than Bcl2-Tg (Fig. 1). Both strains developed lymphadenopathy and splenomegaly similarly. No different microscopic finding was noted in lymph nodes, spleen, or bone marrow (BM). No additional malignancy was found in Bcl2-Tg/TFL^-/- on autopsy. However, significant body weight loss appeared by 30 weeks in Bcl2-Tg/TFL^-/- but not in Bcl2-Tg (Fig. 3).

To identify what causes earlier death in Bcl2-Tg/TFL^-/-, we carefully examined the phenotypic change of BM lymphocytes. We found a unique B220^-IgM⁺ population in Bcl2-Tg BM, which was not found in wild-type. We speculated that TFL deficiency in this population might drive the deterioration in Bcl2-Tg/TFL^-/-. To identify which mRNA was dysregulated by TFL deficiency, we comprehensively analyzed mRNA expression profiles in B220^-IgM⁺ cells in both strains using cDNA microarray chip. Among several genes upregulated at least threefold in Bcl2-Tg/TFL^-/- than Bcl2-Tg, we paid attention to CXCL13, the mRNA expression of which in Bcl2-Tg/TFL^-/- was 4.19-fold higher than that in Bcl2-Tg (p=0.03). In fact, CXCL13 concentration in BM extracellular fluid as well as plasma in Bcl2-Tg/TFL^-/- showed incredible increase in a logarithmic scale (Fig. 2). As a noteworthy event, body weight loss in Bcl2-Tg/TFL^-/- followed the increase of CXCL13 in plasma by 30 weeks (Fig. 3). To confirm that TFL post-transcriptionally regulates CXCL13 mRNA through the degradation of its 3′UTR, we performed a reporter assay with a plasmid vector containing 3′UTR of CXCL13 mRNA. Co-transfection with a TFL expression vector showed decreased luciferase activity compared to the control. This suggests that TFL directly regulates CXCL13 mRNA via its 3′UTR degradation. This regulation occurs more prominently in B-cell lineage rather than myeloid or T-cell lineage, whereas IL-2 mRNA regulation occurs promiscuously.

CXCL13 secretion was significantly increased in the culture supernatant of BM cells but not spleen cells derived from Bcl2-Tg/TFL^-/-. We further sorted several cell populations, including B220^-IgM⁺ in BM, and cultured them for 96 h. CXCL13 secretion from B220^-IgM⁺ population was increased significantly compared to other populations. Thus, we concluded that B220^-IgM⁺ cells in BM are the main producer of CXCL13 in Bcl2-Tg/TFL^-/-. Loss of TFL-driven attenuation for excessive inflammation in lymphoma-bearing mice could contribute to the short survival. It is of interest whether high plasma CXCL13 directly affects cachexia and early death in Bcl2-Tg/TFL^-/-.

TFL deletion in human lymphoma might contribute not only to malignant transformation but also to a major B symptom, i.e., weight loss. Our findings may open a new window for the predictive factor on the prognosis of B-cell lymphoma and/or new therapeutic intervention by targeting CXCL13.

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