Liver transplantation is a promising procedure for patients with benign and malignant liver diseases. However, these patients are universally plagued by immunosuppression-related complications following liver transplantation, especially acute rejection during the posttransplant period (1–3). Currently, the model for end-stage liver disease (MELD) score has been widely accepted as a fair and objective method for liver transplant allocation, which is based on disease severity (4). The correlation between the preoperative MELD score and the occurrence of acute rejection is uncertain. Jia et al. (5) found that liver transplant recipients (LTR) with rejection had higher MELD scores. Similar results were described in adult-to-adult living donor liver transplantation (6). In contrast, Selzner et al. (7) reported that there were no differences between patients with high or low MELD scores in terms of acute rejection. Actually, different circulating lymphocyte subsets have been repeatedly reported as critical components in acute graft rejection (8–11). Therefore, the correlation could be further investigated between the MELD scores and the lymphocytes. In patients with hepatitis B virus-related acute-on-chronic liver failure, the MELD score was found to be correlated positively with the regulatory T cells to T helper 17 (Th17) ratio at the peak point (12), while negatively with the ratio between circulating CD3⁺ T cells and monocytes (13). These patients also had significantly increased CD4⁺ T cells and decreased lymphocytes CD3⁺ T cells than normal subjects (14). Since studies mainly focus on the relations between T cells and the MELD scores, the impact of the MELD score on lymphocytes remains to be elucidated.

Different follow-up periods might have an association with the occurrence of acute rejection, as most acute rejections reported in LTR occur within the first year, especially the first 6 months (15–17). Zhu et al. noted a group of LTR that showed acute rejection at a mean follow-up of nearly 2 months, whose interferon-γ⁺ (IFN-γ⁺) CD4⁺ T cells and interleukin-2 (IL-2⁺) CD4⁺ T cells and transforming growth factor-β (TGF-β⁺) CD19⁺ B cell and granzyme B⁺ CD19⁺ B cell rose significantly compared with LTR without rejection (9). Boix et al. found that LTR, who rejected the allograft, had a statistically significant higher ratio of CD4⁺ CD154⁺ T cells and CD8⁺ CD154⁺ T cells on the 7th and 15th postoperative days (18). Nevertheless, the recommended tacrolimus trough concentrations used in immunosuppressive schemes taper during the first 6 months (19–21). In contrast, patients, who tend to develop a late opportunistic infection, are found to have lower counts of CD3⁺, CD4⁺, CD8⁺ T cells, and natural killer (NK) cells at the first postoperative month (22, 23). Therefore, in an attempt to better control acute rejection following liver transplantation, it is of great importance to know the rationale of the lymphocyte subset shift over time.

Presently, little is known about the dynamic changes of circulating lymphocyte subsets in LTR with the different MELD scores and the follow-up periods. In the meantime, previous studies mainly focused on a specific cell subset and our understanding of how the circulating lymphocyte subsets as a whole respond to a transplant is lacking. Thus, characterizing the shift of circulating lymphocyte subsets in the MELD score follow-up period co-occurrence would help to understand the postoperative immune status and tailor individualized immunosuppressive therapy. We conducted this study to analyze the effects of the MELD score, the follow-up period, and their possible interaction on the lymphocyte subsets following liver transplantation.

In this study, data from LTR with the different MELD scores at the different follow-up periods were collected and analyzed to determine the function of the two factors on the shift of circulating lymphocyte subsets. We found that the MELD score and the follow-up period did not have a synergetic effect on the percentages or absolute counts of lymphocyte subsets. The MELD score affected the percentages of T and TCD4 and the absolute numbers of T-cell subsets and B cells. The follow-up period was in relation to the percentages of T-cell subsets and the absolute numbers of lymphocyte subsets.

Currently, extensive reports have described the shift of a specific cell subpopulation in various diseases (28–30). Nevertheless, there are only limited studies on the changes in lymphocyte subsets under the condition of the MELD score following the liver transplantation. It was found that patients with acute-on-chronic liver failure had high Th17 frequency since the onset point (12) and the decreased T-cell repertoire (31). Freitas et al. reported that patients with benign renal diseases had lower absolute counts of T-cell subsets and B cells when compared with healthy controls (32). These studies reflect that patients with high MELD scores have suppressed immunity. In this study, we also confirmed that LTR with the high MELD scores had lower absolute counts of T-cell subsets and B cells. Since liver failure can lead to a significant decrease in the percentage of CD4⁺ T cells (33), we detected a rather lower percentage of CD4⁺ T cells in LTR with the high MELD scores. In addition, Tanimine et al. (34) demonstrated that chronic liver disease could also impair the potential of intrahepatic NK cells, which is not in agreement with our finding. Of note, dysfunction of NK cells rather than the percentage or the cell number is favored by poor clinical outcomes in liver cancer (35, 36). In this study, circulating NK cells were detected and LTR with both the malignant and benign liver diseases was enrolled. Therefore, this might, at least in part explain that the MELD score failed to affect the percentages and absolute counts of NK cells.

At present, the impact of the follow-up period on circulating lymphocyte subsets following liver transplantation is unclear. Zhuang et al. detected that the cell counts of T, TCD4, TCD8, B, and NK dropped profoundly shortly after stereotactic body radiation therapy and gradually recovered 2 months later (37). Similarly, we observed that the absolute numbers of lymphocyte subsets were reduced in LTR with short-term follow-up. Moreover, we found that the follow-up period affected the percentages of T-cell subsets instead of B cells and NK cells. At our center, basiliximab was used for induction and steroids and tacrolimus were used for maintenance. After the administration of prednisone and tacrolimus, there was a profound lymphocytopenia, a selective decrease in T cells (38–40). Nevertheless, there was no relation between prednisone or tacrolimus trough level and B cells, as mycophenolate mofetil has an impact on the suppression of B-cell functions (41, 42). Therefore, the percentages of T-cell subsets are relatively lower during short-term follow-up. With the rapid recovery of circulating lymphocyte subsets over time, there was a significant rise in the percentages of T-cell subsets.

From the above analysis, we know both the MELD score and the follow-up period can affect the circulating lymphocyte subsets, which show the clinical role in regulating the dose of immunosuppressive drugs. Since the MELD score is calculated based on the parameters of liver function and renal function, LTR with the high MELD scores usually has deteriorated renal function. Additionally, calcineurin inhibitors can further worsen renal function. Exposure to high serum levels of calcineurin inhibitors can result in infection, although the occurrence rate of acute rejection is scarce. Therefore, it is possible to employ a lower dose of calcineurin inhibitors in patients with high MELD scores following the liver transplantation. On the other hand, with the patients' recovery over time, their immunity improves, which makes the patients vulnerable to acute rejection. Hence, the dose of calcineurin inhibitors should increase accordingly. In contrast, current guidelines recommend that tacrolimus trough concentrations are high shortly after surgery, while it is maintained at a relatively low level over time (19–21), which might account for the higher occurrence rate of acute rejection within the first 6 months (15, 16). Taken together, based on the individual immune status instead of empiric therapy, the tailored treatment for each recipient is more favored.

The main limitation of this study is the relatively small number of patients enrolled that may limit the accuracy of our assessment. However, to make the data among different groups comparable, we excluded LTR with any complications. Second, the results represented the experience of a single center in selected patients. Future studies, preferably larger patient cohorts from multicenters, are needed to further confirm our preliminary outcomes.

Liver transplant recipients with either the low MELD scores or the long-term follow-up period are in a relatively activated condition and should be exposed to higher levels of immunosuppressive drugs to prevent immunosuppression-related complications.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by Institutional Review Board of Beijing Chaoyang Hospital. The patients/participants provided their written informed consent to participate in this study.

J-QZ and QH contributed conception and design of the study. FP, R-lW, and SC organized the database. X-LL and Y-nJ performed the statistical analysis. FP and SC wrote the first draft of the manuscript. The rest wrote sections of the manuscript. All the authors contributed to manuscript revision, read, and approved the submitted version. They were accountable for all the aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

This study is supported by the Open Project of Beijing Key Laboratory of Tolerance Induction and Organ Protection in Transplantation (2017YZNS01) and the National Natural Science Foundation of China (81601392).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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