In this retrospective registry analysis of more than 5000 patients with SSc, we demonstrated the efficacy of HSCT with regard to long term overall survival in patients with severe SSc. The standard comparator group A was defined as all patients with dc-SSc. However, there were still significant differences in the distribution of risk factors for progressive SSc between the HSCT and group A. Male gender with diffuse cutaneous skin involvement and positivity for Scl70-antibodies are known to have a high risk of disease related mortality (7). Therefore, we defined an alternative comparator group B by adjusting for these risk factors. Despite our attempt to create appropriate control groups with a comparable distribution of risk factors, there were still significant differences between both control groups and the HSCT group, which is a limitation of this study. Analyses of matched pair controls are planned in the future in order to further homogenize the distribution of risk factors within the HSCT and control cohort. In this study, both control groups comprised a significant proportion of patients with lung and heart involvement, but DLCO was significantly better in group A with all dc-SSc patients but not in the Scl70 + male control group B, indicating that the Scl70 + male control group was a more appropriate comparator for the HSCT group. When we compared the HSCT group with the male Scl70 + group, the HSCT patients showed significant better survival over time. This is in line with the results from three randomized controlled trials, which all showed a significant better overall and event free survival in HSCT treated patients when compared to a cyclophosphamide pulse therapy (10–12).
The benefit of this observational registry is its real life character without the restrictions of in- or exclusion criteria. The long term follow-up of our study and the large number of control patients are the major advantages of a longitudinal registry like DNSS compared to interventional trials.
The decline of patient numbers after 5, 10 and 15 years of follow-up is a limitation of this study. We cannot exclude the possibility that patients with a poor performance status were more likely to be lost to follow-up and the organ function of the remaining cohort seems to improve. However, this scenario seems to be unlikely because our results are consistent with the results from previously reported RCTs (10–11). As absolute patient numbers become very low over time, we focused on the 5 years follow up and here we were able to observe a decline of survival within the first 5 years associated with rapidly progressive disease but not with HSCT or other treatment. Thus, HSCT was not a risk factor for early mortality but reduced the risk for mortality over the first 5 years. In addition, mRSS, as a marker for the skin involvement, significantly decline after HSCT.
With this knowledge and considering the rapid progress in the development of new therapies in SSc, every SSc patient should be referred to an experienced SSc center offering the opportunity to discuss all new therapeutic strategies including clinical trials and HSCT at an early stage of disease. There are several novel treatment options including antifibrotic and specific anti-inflammatory agents in development. However, up to now these treatments cannot replace classic immunosuppressive therapy and HSCT in patients with rapidly progressive disease (10–12).
HSCT protocols have made progress over the years that reduced the risk of TRM and increased the acceptance of this treatment option. Still HSCT should only be performed at an experienced center. Although our patients have been transplanted between 1999 and 2021, we were unable to identify a difference between TRM in the early years compared with transplants in the recent years due to the small numbers of TRM.
Our observational study includes 80 transplanted patients. Therefore, it is one of the largest studies following HSCT treatment in patients with SSc (13). We observed a very low HSCT associated transplant related mortality (TRM) of only 1.3% when compared with previous HSCT trials (10–12, 19). Nine patients received HSCT between 1997 and 2003, which was before the DNSS registry was initiated. Patients who received HSCT in the same period and died before 2003 would not be included in the DNSS registry, which could add as recruitment bias. Since the DNSS is an open registry, we also had patients who were lost to follow-up at some time. The lost to follow-up rate was indicated to be 3.0-3.7% after 5, 10 and 15 years. We cannot clearly differentiate between delayed follow-up reports and real lost to follow-up patients. Thus, we probably underestimate the real lost to follow-up rate to some extent. The delay of reporting deaths could have added to the low TRM in our cohort. Therefore, an HSCT TRM of 1.3% in our cohort was probably underreported. We can only speculate that the real HSCT TRM is likely rather between 1.3% and 5% when selected patients were treated in specialized HSCT centers.
Recently, Spierings et al examined the correlation between HSCT RCT inclusion and exclusion parameters with survival as an outcome in an observational SSc cohort not treated by HSCT, but standard immunosuppression (20). They showed that patients who meet the RCT inclusion criteria had a particularly poor outcome. Those patients who also meet HSCT exclusion criteria like age, pulmonary hypertension, poor kidney function or DLCO < 40% had an even worse survival (20). This defines a group of patients with poor prognosis who might have benefitted from HSCT treatment, but also a group with even worse prognosis in which treatment options are very limited and for which there is a high unmet need. Further studies of therapeutic options for these high-risk patients are warranted.
Another issue is the definition of early progressive SSc, which should be considered for HSCT. Between the late 1990 years and 2012 TRM rates between 10–17% were reported (11, 19). After 2012 and probably due to standardized selection of SSc patients, the TRM rates were around 7% (11, 19). Selection to early SSc means an SSc duration of less than 2 years and clinically progressive skin sclerosis and lung fibrosis before HSCT. Patients with longer SSc duration were considered to accumulate advanced lung fibrosis, heart involvement and pulmonary arterial hypertension that would contribute significantly to a higher TRM after 4 years of SSc duration. Our data show that HSCT is feasible up to 7 years after onset of SSc and TRM is not increased in these patients. Our data also show that selection of SSc patients in HSCT referral centers, reduced toxicities of mobilization and conditioning regimens and advances in the supportive care probably contribute to lower TRM rates below 7%.
In summary, our results on long term survival after HSCT from our large DNSS SSc registry demonstrate a benefit of HSCT in a selected patient cohort with a high risk for rapidly progressive disease. Here, Scl70 positivity, dc-SSc and male gender were the evaluated risk factors but there are even more risk factors that we know from other studies (7, 21–22). Further analyses of risk factors in early SSc disease should define those patients who probably benefit the most from an early intensive intervention like HSCT. Further analyses should also compare the risk of HSCT TRM with the risk of patients with early progressive SSc who meet exclusion criteria for HSCT treatment.