Discussion
To the best of our knowledge, this was the first cohort study assessing the prognostic role of vitamin D in secondary HLH. We discovered that 49 (90.7%) sHLH suffered VD insufficiency (25-(OH)-D<50nmol/L). Low 25-(OH)-D level was associated with survival of sHLH after adjusting for other prognostic factors.
Vitamin D, one of the indispensable elements of health and disease prevention, is mainly synthesized by the skin during sun exposure, or ingested in daily diet. Measurement of serum 25(OH)D was capable of represent the overall level of vitamin D in vitro 20. According to previous research, vitamin D insufficiency is a widespread healthy issue, more common in Asia21. A large multi-center study (2173 healthy adults in Dalian, Beijing, Hangzhou, Guangzhou, and Urumqi) showed that 55.9% populations had insufficient vitamin D—serum 25(OH)D levels were less than 20 ng/ml22. In immune-related diseases, the proportion of VD insufficiency is higher or more serious. Schundeln et al23 reported that the incidence in hemolytic anemia was 80.5%. 80% of patients with multiple myeloma have 25-(OH)-D<50nmol/L24. Consistent with those studies about vitamin D in different diseases, VD insufficiency incidence rate was over 90% in HLH and 71.1% patients were assigned to vitamin D deficiency (<30nmol/L), suggesting vitamin D may be the cause or rather a consequence of the disease. The result of a meta-analysis showed circulating 25-(OH)-D contributed to the occurrence of lung cancer, with increasing 10 nmol/L dose associated with 8% reduction in the risk25.
The biological effects of vitamin D ranged from maintaining serum calcium and skeletal homeostasis to modulating immune processes26. The latter may be one of the reasons related to the prognosis of VD in sHLH. So far, the specific mechanisms of HLH remains no consensus, but more researches considered that congenital and acquired immunity contributed to the occurrence and development. The excessive activation of CD8+lymphocytes and macrophages resulted in wild secretion of pro- and anti-inflammatory cytokines, like IFN-γ, IL-6, IL-12, IL-16, IL-18, TNF-a, and IL-10; those in turn to trigger persistent activation and cause the organ dysfunction characteristics27. Reichel et al 28 showed that the biologically active metabolite of vitamin D3 participated in modulating lymphocyte function via suppressing IL-2 secretion and decreasing production of IFN-γ by normal human peripheral blood lymphocytes. In addition, vitamin D is known to exert several immunomodulatory effects on cytokines synthesis in vitro, as it inhibits TNF-α, IL-12, and IL-17 and stimulates IL-10, promoting a shift from a Th1 to a TH2 phenocyte29. Furthermore, it increased the expression of regulatory T lymphocytes with a skewing away from the Th17 phenotype. It’s noteworthy that 25-(OH)-D show a deeper immunoregulatory effect after lymphocyte activation30. Therefore, 25-(OH)-D was considered to exert the beneficial effect on protecting the risk of immuneregulated diseases. In our study, a negative correlation between 25-(OH)-D and TG levels was observed (r=-0.34, P =0.011), while the increase in TG in sHLH was mostly attributed to IFN-γ. High levels of IFN-γ increase very low-density lipoprotein and decrease lipase activity in the posterior liver, leading to TG clearance defects and hypertriglyceridemia31. 25-(OH)-D was relatively associated with ferritin (P =0.055). Therefore,we speculated that VD may represent the level of IFN-γ. Overall survival was shorter in low 25-(OH)-D level group than that in high level group (28 vs. 125 days P =0.041). After adjusting for hemoglobin, platelet, albumin, TG, β2-microglobulin, ferritin, loge(sCD25), 25-(OH)-D was proved to be an independent protective factor for overall survival (HR=0.364, 95%CI=0.183-0.724, P=0.004). Fattizzo et al30 reviewed that vitamin D deficiency is correlated with disease severity/activity in autoimmune cytopenias. Another plausible pathway reasonable for the relationship between 25-(OH)-D and HLH maybe the mediation of iNK cells. Vitamin D receptor (VDR), the mediator of VD activity, contributed to the development of invariant natural killer (iNK) cells, a subset of lymphoid cells which participates in the host immune response and limits autoimmunity in negative feedback 32.
Second HLH is triggered by various pathologies, lymphoma, infections, autoimmune disorders16. Subgroup analysis showed that in MHLH, vitamin D displayed remarkably ability in predicting survival (HR=0.379, 95%CI=0.187-0.769, P =0.004). Lymphoma accounts for 98.1% of malignant tumors in this study. In diffuse large B-cell lymphoma (DLBCL) and T-cell lymphoma (TCL), Drake et al33 found that the level of 25(OH)D and 1,25-(OH)-D was directly predictive for OS (HR=1.99 95% CI =1.27-3.13 for DLBCL; HR=2.38 95% CI=1.04-5.41 for TCL) and EFS (HR=1.41 95%CI=0.98-2.04 for DLBCL; HR=1.94 95% CI=1.04-3.61 for TCL). Vitamin D can regulate several key cellular processes, including inhibiting carcinogenesis by inducing cell differentiation, inhibiting proliferation and angiogenesis, and promoting apoptosis34. VD also can inhibit the proliferation of lymphoma cell lines and induce their differentiation in vitro experiments35. 25-(OH)-D was proven to influence tumors microenvironment, where chemokines, chemokine receptors, adhesion molecules interacted36. Those may explain the prognostic role of vitamin D in HLH.
Some limitations were present in our study. First, there was a retrospective cohort study and the sample size was small. Then, further clinical researches with more samples are needed to confirm our result. Second, index including proinflammatory cytokines and/or inflammation markers were absent to allow for the assessment of severity. Sfeir et al37 conducted a study about supplying vitamin D and found the improvement of prognosis in lymphoma patients. However, this study did not conclude the supplement of VD and answer whether 25-(OH) D supplement improve the therapeutic effect.