FormalPara Key Summary Points

Why carry out this study?

The use of immunosuppressants is strongly associated with the risk of infection, which has higher morbidity and mortality in patients with systemic lupus erythematosus (SLE).

The association between mycophenolate mofetil (MMF) and infection in patients with systemic lupus erythematosus (SLE) has not been clarified.

The aim of this study is to investigate the association between MMF and the risk of infection, as well as the risk factors that influence the association.

What was learned from the study?

MMF was significantly associated with the risk of overall infection [adjusted odds ratio (OR) 1.90, 95% confidence interval (CI) 1.48–2.44] and different types of infection with different risk OR values.

The infection risk with MMF is higher when the patients with SLE are older, have comorbidities, have been using MMF for the long term, or were on inappropriate combinations of other immunosuppressive agents.

Introduction

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with a wide range of clinical and serologic manifestations that can lead to serious complications and is more prevalent in women [1]. Infection is one of the most serious complications, as well as an important cause of morbidity in patients with SLE. Followed by lupus flares (50.6%), infection (36.1%) is the second cause of hospitalization for SLE [2]. Moreover, infection, especially severe infection (SI), has been the most common cause of death (up to 65.8%) in SLE [3], and infection-related deaths are more common in Asian patients globally [4]. Patients with SLE are susceptible to infections because of their own intrinsically dysfunctional immune systems, which is exacerbated by disease activity, but also in close association with immunosuppressive therapy [5].

Mycophenolate mofetil (MMF) is an important immunosuppressant that is widely used for autoimmune diseases and is recommended as a first-line medication for the induction and maintenance of remission in patients with SLE with lupus nephritis (LN) [6]. Due to its lymphocyte specificity, MMF is a favorable therapeutic option, especially for patients who cannot tolerate the severe adverse effects of cyclophosphamide (CYC) [7, 8]. However, the side-effects profile of MMF has not been extensively evaluated, especially for infections. In multiple evaluations of small samples, the incidence of MMF-induced infections ranged from 16 to 44% and varied with the length of follow-up, second only to gastrointestinal reactions, although there were few serious infections requiring hospitalization [9,10,11]. A recent real-world study in Japan followed 452 adult patients with LN taking MMF for 6 months showed that the most common adverse effects of MMF were diarrhea, herpes zoster, and cytomegalovirus infection, and the degree of adverse effects appeared to be related to the MMF dose [12]. Several meta-analyses in the efficacy and safety of MMF compared with CYC in the treatment of LN showed controversial results. The overall infection rate was observed to be lower for MMF than for CYC in some studies [13,14,15] but was not significantly different in others [16,17,18]. In addition, in a recent analysis of predictors of hospitalized infection in multi-racial patients with SLE receiving immunosuppressive agents, the positive association was only significant for CYC use, but was not significant for MMF use [19].

So far, no previous studies have comprehensively evaluated the spectrum of infection after MMF use in SLE, and the risk of infections caused by MMF and its combination with other immunosuppressive agents remains unclear. Based on a Chinese cohort of hospitalized patients with SLE from 11 years of electronic medical records, a nested case–control study was conducted to evaluate the association of MMF use and its combination with other agents with different types of infections.

Methods

Study Design and Participants

This study was approved by the Institution Review Committee of Nanjing Drum Tower Hospital (No: 2022-220-01). Written informed consent was exempt from ethics approval requirements according to national legislation from National Health and Family Planning Commission of China [20]. A longitudinal database for a cohort of patients with SLE was established from the medical record system of the Affiliated Drum Tower Hospital, Nanjing University Medical School. The subjects were hospitalized patients with SLE from January 1, 2010, to December 31, 2021, who met at least four American College of Rheumatology (ACR) diagnostic criteria for SLE [21]. Basic demographic information, clinical manifestation, laboratory test data, discharge diagnosis, comorbidities, and medication information were collected from both inpatient and outpatient medical records. Disease activity of patients was evaluated using the SLE Disease Activity Index 2000 (SLEDAI-2 K) criteria. The exclusion criteria for the study group were as follows: (1) patients with only once medical visit or missing medication history records, (2) patients with pre-existing malignancy and human immunodeficiency virus (HIV) infection before and at baseline.

In this study, data were analyzed using a nested case–control study. The study population was divided into an infection group (cases) and a control group according to whether there was any type of infection in the discharge diagnosis. Patients could enter the study only once when they first met the inclusion/exclusion criteria, and the earliest record should be the priority when patients with more than one hospitalization record (especially within 30 days), to avoid bias in the results from medical duplication of documentation about the same infectious event or residual effects of prior serious infections on the patient’s overall health.

Exposure

MMF use was the exposure of interest. The definition of MMF use and all other medications, clinical manifestations, and comorbidities were the presence within 1 year before diagnose infection or the end of follow-up. The last daily dosage and duration of MMF taken were collected. Other treatments during this period included mean daily glucocorticoids (GCs, and all classes of GCs were converted to equivalent prednisone doses), hydroxychloroquine (HCQ), CYC, tacrolimus (TAC), and the combined other immunosuppressants, including leflunomide, methotrexate, cyclosporin A, azathioprine, thalidomide and tripterygium glycosides.

Outcomes

We identified infections using hospital discharge diagnosis codes (Supplementary Materials Table 1). The primary outcome was overall infection defined as any of the infections identified in our study. Common hospital-acquired infections, such as catheter-related infections, peritoneal dialysis infections, and surgical site infections, were not included in the findings, as we instead focused on infections that were most likely to warrant hospitalization rather than to occur as a result of hospitalization. Three secondary outcomes, including bacterial infections, opportunistic infection, and viral infection, were classified according to discharge diagnosis classification, combined with whether intravenous antibiotics were used to determine bacterial infection. The pathogen’s result is determined through laboratory culture. If it cannot be identified, the infection is classified as being of unknown cause. This typically applies to mild types of infections, such as upper respiratory and skin infections. The causative agents among bacteria included staphylococci, Haemophilus, etc. Opportunistic infections included tuberculosis, non-tuberculous mycobacterial infection, and systemic fungal infections (aspergillosis, cryptococcosis, candidiasis, histoplasmosis, pneumocystosis). Viral causative agents include herpes zoster virus (HZV), human cytomegalovirus (HCMV), Epstein–Barr virus (EBV), and human papilloma virus (HPV). Infections are also classified according to the site of involvement and included upper respiratory tract infection, urinary tract infection/pyelonephritis, skin and soft tissue infection, meningitis/encephalitis, pneumonia, cellulitis, viral hepatitis, endocarditis/myocarditis, septic arthritis/osteomyelitis, and septicemia/bacteremia. Patients are classified as single infection when they have a single pathogen or site infection during this hospitalization, otherwise, they are considered as a complex infection.

Statistical Analysis

Differences in covariates between the infection group and the control group were compared. According to normal distribution, continuous variables were described by the mean and standard deviation (SD), or median and interquartile ranges (IQR), and were compared using Student’s t tests or Mann–Whitney U tests. Categorical variables were compared using the χ2 test. A multivariate unconditional logistic regression model was used to calculate the odds ratio (OR) and corresponding 95% confidence interval (CI) for the association between MMF use and the risk of infection. Stratified analyses were conducted based on factors such as sex, age, SLE disease period, comorbidities, organ involvements, and combined medications. Dose–response relationships between MMF use and risk of infection and the potential multiplicative interactions between MMF and other drugs including CYC, TAC, GCs, HCQ, and other immunosuppressants were assessed. The ratio of ORs (ROR) was evaluated for multiplicative interaction by including the main effect variables and their product terms in a multivariate logistic regression model. Multivariable analysis models were adjusted for potential confounders, including sex (female = 1, male = 0), age (continuous), SLE disease period (continuous), SLEDAI score on admission (continuous), hypertension (yes = 1, no = 0), diabetes mellitus (yes = 1, no = 0), mean daily prednisone equivalent dose (continuous), combined with CYC treatment (yes = 1, no = 0), combined with TAC treatment (yes = 1, no = 0) and combined with other immunosuppressants (yes = 1, no = 0). Furthermore, patients in the control group were propensity score matched (PSM) 1:1 by age (years), sex, disease duration (years), and SLEDAI score to patients with infection, using logistic regression analysis with a match tolerance of 0.02. All analyses were performed using SPSS 26.0 software. P < 0.05 was considered a significant difference.

Results

Clinical Features of Patients

A total of 4548 patients with SLE were identified from the Nanjing Drum Tower Hospital medical database. After applying the exclusion criteria (Fig. 1), a total of 3339 patients with SLE were included in the analyses, of which 1577 patients who developed infection were defined as cases and 1762 patients without infection were defined as controls.

Fig. 1
figure 1

Nested case–control study of infection after MMF use developed from a retrospective cohort of patients with SLE. a SLE systemic lupus erythematosus. A longitudinal database for a cohort of hospitalized patients with SLE was established from the medical record system of the Affiliated Drum Tower Hospital, Nanjing University Medical School; the standardized form was used to collect patients’ information from both inpatient and outpatient visits, including basic demographic information, clinical manifestation, laboratory test data, and disease diagnosis medications. b MMF mycophenolate mofetil. The definition of MMF use and all other medications were the presence within 1 year before diagnose infection or the end of follow-up

Full size image

The characteristics of patients with SLE for patients with infection and controls are presented in Table 1. The infection group was older, had higher SLEDAI scores, had more severe clinical manifestations, and had more comorbidities such as hypertension and diabetes. In terms of medication, the infection group received more and different treatments before than the controls.

Table 1 Characteristics of patients with SLE by status of infection
Full size table

The Association Between MMF Use and Infections

Table 2 shows the association between MMF use and various infections. MMF was found to be significantly associated with increased risk of overall infection in patients with SLE, with adjusted OR of 1.90 (95% CI 1.48–2.44). The positive associations were also found for bacterial infection (adjusted OR 2.07, 95% CI 1.55–2.75), viral infection (adjusted OR 1.92, 95% CI 1.23–3.01), opportunistic infection (adjusted OR 2.13, 95% CI 1.31–3.46) and other infections of unknown cause (adjusted OR 1.73, 95% CI 1.21–2.47). The OR value of complex infection (adjusted OR 2.12, 95% CI 1.39–3.23) was slightly higher than that of single infection (adjusted OR 1.89, 95% CI 1.46–2.45). For each specific infection, the top three infections associated with MMF-related risk (using the control group as a reference) were bacteremia/septicemia (adjusted OR 3.16, 95% CI 1.29–7.76), urinary tract infection/pyelonephritis (adjusted OR 3.14, 95% CI 1.94–5.11), and herpes zoster (adjusted OR 2.85, 95% CI 1.32–6.15).

Table 2 Associations between MMF use and risk of infection in patients with SLE
Full size table

In addition, those positive associations between MMF use and overall and specific types of infection were also observed after PSM to match age, sex, duration of disease, and SLEDAI score between patients with infection and controls (Supplementary Materials Tables 2 and 3).

We also conducted stratified analyses of 19 factors such as sex, age, SLE disease period, comorbidities, and therapeutic drugs (Table 3). Adjusted OR for MMF use and overall infection was still significant across most subgroups, but was not significant in male patients, those younger than 18 years old, SLEDAI score at 0–6, and combined with TAC treatment. Notably, the use of MMF was significantly associated with a high risk of infection in patients with diabetes (adjusted OR 6.18, 95% CI 1.32–28.90), central nervous system (CNS) involvement (adjusted OR 3.93, 95% CI 1.43–10.80), thrombocytopenia (adjusted OR 3.75, 95% CI 1.98–7.12), and age > 55 years (adjusted OR 3.40, 95% CI 1.31–8.78).

Table 3 Risks of overall infection in subgroups of patients with SLE treated with MMF
Full size table

Dose–Response Relationships Between MMF and Infections

The associations between different dosage or lengths of MMF use and infections are presented in Tables 4 and 5. The median dosage of MMF last recorded was 1 g/day, and the highest dosage was 2.5 g/day. The risk of overall infection was positively associated with MMF dosage (g/day), with adjusted OR of 1.48 (95% CI 1.23–1.79). A strong dose–response relationship was observed between the lengths of MMF use and infection development and a greater risk of infection was seen for patients who were treated with MMF in 90–365 days (adjusted OR 3.23, 95% CI 1.83–5.69), and > 365 days (adjusted OR 3.89, 95% CI 2.28–6.64) compared with non-users. The results were also consistent across secondary outcome analyses of different types of infection: bacterial, viral, and opportunistic.

Table 4 Dose–response relationships between the dosages of MMF use and risks of infections in patients with SLE
Full size table
Table 5 Dose–response relationships between the lengths of MMF use and risks of infections in patients with SLE
Full size table

The Combined Effects of MMF with Other Drugs on Infections

The CYC use was positively associated with overall infection and had the same adjusted OR of 1.90 (95% CI 1.53–2.35) with MMF use. In addition, the overall infection was also positively associated with the combined other immunosuppressants (adjusted OR 1.59, 95% CI 1.31–1.93) and daily GC > 20 mg/day (adjusted OR 1.28, 95% CI 1.10–1.49), but was not significant with TAC and HCQ (Supplementary Materials Table 4). The associations between MMF in combination with these agents on overall infection are presented in Table 6. The combinations of MMF with CYC and other immunosuppressants had higher risks of infection compared to single drugs, with adjusted OR of 3.75 (95% CI 2.53–5.56) and 3.51 (95% CI 2.45–5.03), respectively. However, the multiplicative model interactions were not statistically significant. The results were also consistent across secondary outcome analyses of different types of infection: bacterial, viral, and opportunistic (Supplementary Materials Tables 5, 6, and 7).

Table 6 Joint effects of MMF and other major drugs on overall infection in patients with SLE
Full size table

Discussion

In this nested case–control study of patients with SLE, we found that MMF use was positively associated with subsequent overall infection, as well as bacterial infection, viral infection, and opportunistic infection. Moreover, dose–response relationships were observed between MMF use and the risk of infections. In addition, we showed the associations between MMF use and risk of infection in different groups.

As previously studied, the use of immunosuppressive drugs increases the overall risk of infection in patients with SLE [5]. While MMF, as a potent immunosuppressive agent, is less likely to cause serious adverse effects associated with other immunosuppressive drugs, such as nephrotoxicity, hepatotoxicity, and neurological disorders [22], infection remains one of the major adverse effects. The RELES cohort analyzed how predictors of serious infection changed during the first 2 years of follow-up in 282 patients with SLE, and in both the first and second years of disease MMF treatment was a significant factor [23]. In the present study, MMF use was also significantly and positively associated with the occurrence of overall infection in patients with SLE, with a high incidence of 67.6% in MMF users and 44.7% in MMF non-users, which is in line with previous findings [24].

In the secondary outcome analysis, the magnitude of the associations between MMF and bacterial, viral, and opportunistic infection were almost the same. The top three infections associated with MMF were bacteremia/septicemia, urinary tract infection/pyelonephritis, and herpes zoster, respectively. Consistent with our findings, many previous studies have reported increased bacterial, opportunistic, and viral infections in patients with SLE with MMF, respectively, mostly from retrospective studies and case reports with limited sample sizes. In the Hopkins Lupus Cohort, 244 patients newly started on MMF were found to have a statistically significant increased risk of bacterial infection, although leukopenia was not exacerbated, at a median follow-up of 47 days by the next visit [25]. Similarly, there is also an increased risk of viral infection after taking MMF, including HPV [26], and in particular the incidence of herpes zoster, increased 26-fold after adjustment [27]. In our study, the results were significant but not that high. In addition, there are several retrospective studies and case reports from small samples of MMF that report an increase in opportunistic infections such as Pneumocystis jirovecii pneumonia in patients with SLE [28, 29], and the positive associations were borderline between MMF use and upper respiratory tract infection and tuberculosis here, while some infections in our study were not associated with MMF, including encephalitis/meningitis, endocarditis/myocarditis, septic arthritis/osteomyelitis, HCMV infections, HPV infection, and virus hepatitis, probably because of the small number of cases in these subgroups. Studies in kidney transplant recipients have found that tissue-invasive HCMV disease occurs more frequently in patients treated with MMF [30, 31]. However, while the ability to extrapolate such findings to current preemptive approaches to HCMV infection in patients with SLE is limited, we could not find a statistically significant difference between HCMV infection and the use of MMF; there was only a slight increase. Interestingly, MMF has been shown to increase the anti-herpesvirus and anti-cytomegalovirus activity of ganciclovir, acyclovir, and penciclovir by 350-fold [32]; however the combined effect of MMF and antiviral drugs needs to be verified clinically in further studies.

MMF is often used in severe patients with SLE, who have the more active disease with major organ damage, and was also in combination with other drugs such as corticosteroids. Studies have reported that general risk factors that predispose patients with SLE to infection include essential features such as advanced age, ethnicity, history of smoking, and history of chronic disease [33]. In stratified analyses, we also found that the use of MMF was significantly associated with an increased risk of infection in patients with age > 55 years, diabetes, CNS involvement, and thrombocytopenia, etc. For those patients having a high risk of infection, MMF should be used with caution in lower doses and monitored for infection. The 2019 edition of the European League Against Rheumatism (EULAR) guidelines for the management of SLE [34] suggested that disease-related risk factors for the development of infections in patients with SLE include disease activity, GCs (prednisone dosage above 7.5–10 mg/day, and high-dose methylprednisolone shock therapy), immunosuppressive agents, and the use of biologics. After adjusting for those confounding factors, this study found that infection was positively associated with CYC, MMF, other immunosuppressants, and the GCs (> 20 mg/day), but not TAC and HCQ. Consistent with previous research, we found that MMF was almost as associated with infection risk as CYC. In a national longitudinal study of US patients with SLE at high risk of infection, there was no difference in the rate of serious infection and mortality between the new use of MMF, CYC, or azathioprine [35]. A meta-analysis including a total of 725 patients from seven trials comparing the adverse events of MMF with CYC found no statistically significant differences in infection and gastrointestinal symptoms between the two groups [16]. Furthermore, this study found a higher risk of infection than single agents, when MMF was be used in combination with CYC and other immunosuppressants; however, MMF is relatively safe in combination with TAC, GCs (< 20 mg/day) and HCQ. Previous results have also shown that TAC does not appear to be an infection threat [36, 37], and it is safe and more effective in combination with MMF [38]. HCQ is an antimalarial drug. The British Society for Rheumatology (BSR) guidelines for the management of adult lupus [39] state that HCQ needs to be emphasized as a base combination drug for the treatment of SLE, thereby reducing the use of GCs and other immunosuppressive agents and thus reducing the risk of treatment-related infections.

The reversible cytostatic effect of MMF allows for dose adjustment or discontinuation to prevent severe toxicity or excessive suppression of the immune system [22]. The current recommended MMF dosage in Asians during induction therapy is 1.5–2 g/day [40]. Low-dose MMF at 0.5–1 g/day was verified with good efficacy and tolerability in Chinese patients [41], while higher dosage of up to 3 g/day increase infection-related mortality in Asians [42]. In our study, the median dosage of MMF was 1 g/day and the highest dosage was 2.5 g/day for SLE treatment, and the results showed that the risk of infection was positively associated with increased dosage, although OR of infection for MMF dosage above 1 g/day group was not higher than for less than 1 g/day, which might be due to information loss when continuous variables are converted to rank variables. Two possible mechanisms may be the explanations for the association between MMF use and development of infection. First, as a strong immunosuppressive medication, MMF may impair the immune function of patients with SLE. Although these medications are beneficial for SLE disease control, the suppression of immunity may predispose these patients to all kinds of pathogen infection [43]. Second, MMF may predispose patients to infection by affecting their hematological system. MMF has been reported to cause decreases in leukocyte counts in patients with SLE at different frequencies [18, 44], while overall reduction of white blood cells was not found to be predictive of infections in a previous study [45]. More studies are needed to investigate the relationship between these factors.

There are several limitations to this study. First, data were analyzed based on a single-center prospective SLE cohort database. Second, a possibility of selection bias may exist, since the study recruited hospitalized patients with SLE only, which leaves a large proportion of patients with SLE on MMF who do not get hospitalized. The results and the projected risk of MMF thus may be overestimated. Third, the association between MMF use and some specific infections cannot be observed, since the number of secondary outcomes was relatively small. Fourth, we did not examine the risk of recurrent infections. However, as a real-world study, it can compensate for the shortcomings of traditionally designed studies, obtain various types of patients, have more generalized results, and provide widely relevant evidence. In addition, this study used rigorous pharmaco-epidemiological approaches to provide findings with high validity by both multivariate logistic regression models and stratified analyses to control potential confounders. This study was the first to comprehensively clarify the associations between MMF use and the risk of infection and several specific infections in patients with SLE, and give evidence for high-risk situations to arouse clinical vigilance. The findings can help in clinical guideline for better use of immunosuppressants to reduce the risk of infection under the premise of controlling disease activity and preventing relapse.

Conclusions

In conclusion, this study found that the use of MMF was associated with an increased risk of infection in patients with SLE, regardless of bacterial, viral, opportunistic, single, or complex infection, and the overall risk of infection is no less than CYC. Moreover, a higher risk of infection was observed in longer MMF users, elderly, those with some comorbidities such as diabetes, CNS involvement, thrombocytopenia, and those in combination with other medications, such as CYC or other immunosuppressants. The study's findings justify the careful utilization of MMF to closely monitor infections among these patients. Lower doses or discontinuation should be considered when appropriate, although the importance of MMF as a steroid-sparing agent should not be underestimated.