Severe SARS-CoV-2 infection induces a cytokine storm, leading to ARDS and MOD, which are very serious health conditions.Those characteristics of COVID-19 make disease controlchallenging if using a single strategy.MSC properties allow the systemic distribution of positive immunomodulatoryand regenerative effects throughout the body, thereby ensuring a systemic effect in addition to local modulation67. To our knowledge, this is the first report that presents the longest follow-up after MSC treatment in COVID-19 patients. This trial was conducted during the early stages of the COVID-19 outbreakand proposed UC-MSC infusion as an adjunctive therapy in the recovery and postacute sequelae reduction caused by COVID-19. The results indicate tolerability and safety, and suggest the efficacy of UC-MSC infusion in critically ill patients.
For MSC therapy to be feasible in patients with critical illnesses such as COVID-19, cells must be obtainable within a very short time and in adequate numbers from a reproducible production process68. In this context, the use of allogeneic UC cells, available in a Biobank, allows postthaw, culture recoveryand infusion within 72 hours after the patient's inclusion in the study. Some studies69,70 show that MSCs need to restart their metabolism and other biochemical processes before infusion, recovered by a 24–72hours subculture, because thawed MSCs seem to be unresponsive directly after thawing. If MSCs are allowed to recover in culture, they restore their functionality. For this reason, in this trial, fresh cells were used in all infusions.
Sample randomization was conducted in this study to protect against imbalance in biasing caused by enrollment. However, with small numbers of patients, there is still potential for imbalance. Some differences between groups were observed at baseline, demonstrating that the UC-MSC group seemed to be more compromised than the placebo group.
All patients received standard treatment with steroids because it is recommended to use steroids in crically ill patients to inhibit the inflammatory response, especially for those requiring respiratory support71. Glucocorticoids, including dexamethasone, also reduce neutrophil extracellular trap (NET) formation, most likely by suppressing the expression of inflammatory mediators that activate neutrophils72. Heparin was also used as a standard treatment because it reduces NET formation73, as it has been shownto have therapeutic value in COVID-19 treatment74.
In this research, two patients in the UC-MSC group had no previous chronic diseases, while all other patients had comorbidities. This is in line with studies that show that the presence of underlying conditions such as cardiovascular disease, chronic pulmonary disease, and diabetes are risk factors that will require critical care75.
The interval between the first infusion and hospital discharge was similar between groups (average of 16.3 days for the UC-MSC group and 14.4 days for the placebo group), showing that MSCs do not accelerate patient recovery. Similar results were observed by Adas et al.50, when compared the control and experimental groups of criticaly ill patients. The average length of stay was 45 days in the control group and 47 days for the placebo group, with no significant differences.
After three IV UC-MSC infusions, none of the patients developed a thromboembolic event, and only mild AEs resolved naturally in a small number of cases. These AEs are unrelated to cell infusion, showing safety with no life-theatening complications. Safety was shown in other studies that also performed MSC-based interventions. They did not noticed any acute infusion-related issues, alergic reaction, delayed hypersensitivity or secondary infections in the patients either39,45,47,48,54,76,77.
The mortality rate in this study was 35%. Out of six patients, five had at least one comorbidity, and the most common cause of death was secondary bacterial infection. A similar study carried out in Brazil showed that in the same period of time, the mortality rate in ICU patients in theSouth region was 53%78. Hashemian et al.52 studied critically ill patients with severe hypoxemia who requiredMV, and observed a 45% mortality rate- most of which had signs of multiorgan failure or sepsis, and died 5-19 days after the first infusion. Also Dilogo et al.51 showed that the mortality rate was 65% in a group of intubated critically ill patients with COVID-19 in the ICU, and higher mortality was associated with patients who had two or more comorbidities. Another trial with critically ill patients was conducted by Adas et al.50, and they observed 33% mortality in patients who received MSC therapy. The most common cause of death was secondary infections due to bacteria, followed by myocardial infarction and thromboembolism. Data fromsuch studies reinforce that critically ill COVID-19 patients had small chances of survival.
Gender was also shown to be related to mortality. The ACE2 gene localizes on the Xchromosome, and ACE2 levels in the blood are higher in males than in females as well as in patients with diabetes or cardiovascular disease79-89. In this study, it was observed that six patients (83.3%) were male. Therefore, male patients might be more likely to die from COVID-19 because of the high expression of ACE2.
A high mortalityrate of patients who presented kidney dialysis dysfunction was also observed83. The incidence of acute kidney injury (AKI) secondary to COVID-19 is high84. Kidney failure appears to occur late in the course of disease and is strongly associated with high mortality among hospitalized COVID-1985,86.Ghonimi et al.87 reported a strong association between death related to COVID-19 infection in dialysis patients, which was also observed in the study of Costa et al.88, showing that COVID-19 patients with AKI who need dialysis had worse outcomes. Lino et al.89 also showed that a worse prognosis is frequently associated with a more rapid evolution to intensive and respiratory care or even dialysis88.
According to Saleh et al.53, the optimal time for cell infusion is the second week of the disease, namely, the second phase, where there is hyperinflammation beginning on days 7 to 15. Zhu et al.56also suggested that MSCs can improve the outcome of patients with severe/critical symptoms more significantly than common/mild patients. In this trial, only critically ill patients were included, and the average interval between symptom onset and first cell injection was 10.7 days for the UC-MSC group and 12.1 days for the placebo group. Possibly due to a combination of suitable patient and the ideal time points for patient treatment, an overall patient benefit after IV UC-MSC infusion was noticed.
Verifying the presence of genomic material of the virus in serum or plasma represents a useful approach to evaluate the impact of the extrapulmonary dissemination of viral material on disease severity and on the host response to the infection90,91. The systromalic dissemination of the virus or viral components is associated with the severity of COVID-19 and with a number of parameters indicating the presence of a dysregulated response to the infection92. In this study, in both groups, 14 days after infusion, there was a reduction in viral load over time, without significant differences. This is in line with Lanzoni et al.46, who showed no differences between the UC-MSC treatment and the control group. According totheauthors, UC-MSC treatment seems to be more closely associated with a decrease in inflammatory cytokines rather than a change in viral load.Leng et al.39 also observed that critically severe patients became negative for hCoV-19 nucleic acid 13 days after transplantation.
In the acute phase reaction of an inflammatory process, there is a variation in the concentrations of various plasma proteins, including C-reactive protein and ferritin. They are important biomarkers of inflammation in the context of COVID-19 progression because they are predictive of in-hospital93,94. Patients with the highest ferritin levels also presented significantly higher levels of C-reactive protein and serum creatinine89. In this study, analysis of inflammatory markers showed that C-reactive protein and ferritin values, in the UC-MSC group, decreased in the second and fourthmonths compared with baseline. In the placebo group, the levels were always higher than the reference ranges. Those results are in accordance with studies that show that the inflammatory biomarkers were increased in COVID-19 patients95,96.
Critically ill COVID-19 patients have a CRS storm that involves elevated levels of circulating cytokines and immune-cell hyperactivation97. It occurs due to the combination of a defective (or delayed) first line of defense, followed by persistent hypercytokinemia and a dysfunctional Tcell response. That results in impaired clearance of apoptotic cells or infected/activated macrophages, followed by multiple cytokine release, hemophagocytosis, coagulopathy, and ARDS98-100. The clinical manifestation is the sharp rise of a large number of cytokines within a short time frame. Liu et al.101identified that serum levels of IL-6 (>32.1 pg/mL), one of the mediators of hyperinflammation, have a significant correlation with the severity of COVID-19, and can be used to predict disease risk. As part of this study, analysis of plasma cytokine levels was performed, andincreased IL-6 levels were observed in both groups. However, in the UC-MSC group, there was a significant reduction from day 14 (mean 9.59 pg/mL in the second month and 3.70 pg/mL in the fourth month), while in the placebo group, the levels remained high (mean 45.97 pg/mL at the second month and 100.97 pg/mL at thefourth month). A gradual decline in IL-6 levels, as shown in the present study, might be a biologically relevant marker of the efficacy of UC-MSC treatment in patients with COVID-19.
Levels of IL-8 in the UC-MSC group until the fourteen day were always higher than those in the placebo group. In both groups, there was a large reduction in values at 2 and 4 months. IL-8 is known as a neutrophil chemotactic factor and plays a major role in the recruitment of neutrophils to the site of infection. Ma et al.102 did not observe any association of IL-8 concentrations with the severity of COVID-19, butdid observe an association between IL-8 serum levels and the duration of illness in patients with severe COVID-19. Thus, IL-8 will be a signaling pathway in the evolution of COVID-19. This same result was obtained by Li et al.103, who showed that serum levels of IL-8 correlated to the overall clinical disease scores at different stages of the same COVID-19 patients. Hence, IL-8 may act as a biomarker for COVID-19 disease prognosis. The higher levels in the UC-MSC group enforced that patients in this group had a worse prognosis than those in the placebo group. When there was patient recovery, in the second month, there was a reduction in IL-8 levels in both groups.
The MCP-1-CCL2 chemokinehas a critical role in the process of inflammation, where it attracts or enhances the expression of other inflammatory factors/cells. It is a biomarker associated with the severity of COVID-19 disease and can be related to the risk of death in COVID-19 patients104.The MCP1-CCL2 chemokine level in the UC-MSC group, until the fourth day, was higher than that in the placebo group, decreasing from the fourteen day to the fourth month, reaching levels with no differences in relation to the placebo group. Resultsreveal that there was decreased inflammation and clinical improvement in these patients after cell treatment.
There were no differences in IL-7 levels in the UC-MSC group at any evaluation time; on the other hand, in the placebo group, the IL-7 level was decreased in the fourth month, with a significant difference compared to baseline. IL-7 is a pleiotropic cytokine essential for lymphocyte survival and expansion. Most likely, for this reason, a recovery in the TCD3, TCD4 and NK lymphocyte numbers was not observed in the placebo group at different evaluation times. IL-7 promotes lymphocyte expansion and possibly reverses T-cell exhaustion and may be useful in restoring immune systemic homeostasis105. Studies show that IL-7 exerts antiapoptotic properties and induces potent proliferation of naive and memory T-cells, leading to replenishment of circulating TCD4+ and TCD8+ 106,107.
At the inflammatory stage, there is a discharge of cytokines, chemokines and growth factors triggering neutrophil and monocyte recruitment108. Neutrophils and the imbalance between NET formation and degradation play a central role in the pathophysiology of inflammation, coagulopathy, organ damage, and immunothrombosis, which characterize severe cases of COVID-19109. Therefore, some clinical studies have found that the number of neutrophils in the bronchoalveolar lavage fluid of ARDS patients is correlated with the severity of COVID-19 and the cytokine storm110. Activatedplatelets form aggregates with leukocytes, particularly in patients with severe disease111,112. Circulating platelets bind neutrophils and may result in NET formation in the pulmonary and renal microcirculation109, thereby contributing to immunothrombosisin patients with COVID-1990. Zhu et al.56 found that MSC treatment can reduce plasma NET-DNA levels in COVID-19 patients. In this research, in the second month after treatment, there was a reduction in the number of neutrophils to the reference range in the UC-MSC group, while in the placebo group, the values were always above the normal range. The number of platelets was at the reference range in both groups in all evaluations. However, in the second and fourth months, in the placebo group, there was an increase in values above the reference. The decreased number of neutrophils and normal platelet values indicate a lower risk of thrombosis in the UC-MSC group.
D-dimer, a fibrin degradation product, is also used as a biomarker for thrombotic disorders and has been identified as a potential indicator for prognosis in COVID-19 patients113,114,115. According to this study, both groups presented D-dimer values above the reference in all evaluations. However, in the UC-MSC group, a decrease in D-dimer values was observed in the second month. This indicates that cell infusion was effective in reducing D-dimer levels in the UC-MSC group, decreasing the risk of thrombosis formation in these patients.
Lymphopenia is a typical profile in patients with COVID-19116,117and might be related to disease severity and mortality118,119; therefore, it is very important to determine these parameters when evaluating critically ill patients. In this trial, the number of TCD3 and TCD4 lymphocytes was lower in the UC-MSC group than in the placebo group at baseline and day 2. The number increased at thesecond and fourth months compared to baseline. The number of NK cells in the UC-MSC group was higher in the fourth month than at baseline. These results are in line with studies that have shown that all subsets of lymphocytes were decreased in COVID-19 patients116,118,119,120 and that T-cells exhibit elevated exhaustion levels and reduced functional diversity116,121,122. Patients with a severe form of COVID-19 have fewer multifunctional and nonfunctional CD4+ Tcells and fewer nonexhausted CD8+ Tcells than patients with mild COVID-19122. Several studies observed that lymphocyte count returned to the normal range in the experimental group, and the time was significantly faster after stromal cell infusion compared with the control treatment48,123,124.
The persistent follow-up of discharged patients with COVID-19 is essential to find ways to improve quality of life and reduce morbidity and mortality by efficient prevention. In this study, some markers of cardiac and kidney function were evaluated,and a CTscan was performed for pulmonary evaluation. In this trial, no differences were observed in relation to troponinI levels, corroborating the results observed by Johnsen et al.23, who analyzed patients with long COVID-19 sequelae three months posthospitalization and observed no signs of cardiac dysfunction.
Huang et al.25showed that 13% of patients without acute kidney injury at the acute phase had a decreased glomerular filtration rate at follow-up, exhibiting an underestimation of patients with kidney dysfunction. Persistent impairment in renal function can occur following an episode of acute kidney injury, with the potential to progress to end-stage kidney disease with dialysis125, which highlights the importance of long-term follow-up. Creatinine values in the UC-MSC group were above the reference until thesecond month, followed by a reduction at the fourth month. In the placebo group, there was an increase above the reference at the second and fourth months. It is probable that kidney lesions acquired during the disease’s activity remain sequelae that may result in a slow and asymptomatic progression toward advanced stages and chronic kidneyfailure (CKD). Thus, patients who have recovered from COVID-19 who present proteinuria, hematuria, elevated creatinine and AKI should be monitored for CKD126. Increased creatinine values may also be associated with the patient's nutritional status; however, these patients underwent nutritional assessment at 2 and 4 months, and it was observed that all were in good nutritional status.
The benefits of corticosteroid treatment for accelerating the recovery of lung injury,according to pulmonary function assessment and chest imaging in patients with COVID-19, are controversial71,125,127. Therefore, new strategies to avoid pulmonary sequelae need to be developed. Once injected intravenously, a significant amount of MSCs accumulate in the lungs, and they secrete numerous factors that play an important role in immunomodulation, protect alveolar epithelial cells, restore thepulmonary alveolar niche, prevent fibrosis, and improve overall pulmonary function, which is a great benefit for treating severe pulmonary disease in COVID-1948,128.In addition, lung function and chest CT changes may be impaired months after the infection129. Huang et al.25 observed that a considerable proportion (22–56%) of patients had a pulmonary diffusion abnormality 6 months after symptom onset. In this trial,there was a decrease in lung lesion extension in the UC-MSC group after 4 months of follow-up. The improvement of pulmonary lesions directly affects the recovery of lung function and the remission of clinical symptoms48; therefore, the results observed in this study could reflect reduced lung inflammation in the UC-MSC group mediated by immune regulation.
Throughout this trial, there were some limitations such as the patient assessment time, between the fourteen day and 2 month, was very long. Many parameters may have improved before the 2 months, but the exact moment could not be observed. Sample randomization was conducted in this study; however, based on some inflammation markers andlymphocyte subpopulations, the UC-MSC group seemed to be more compromised than the placebo group at baseline. Although the sample size was not large enough to stratify subgroups, it was difficult to exclude bias. The emergency condition in ICUs did not allow us to carry out CT evaluations in all patients at different times.
The results of this study revealed that in the UC-MSC group, there was a reduction in the levels of ferritin, IL-6 and MCP1-CCL2 on the fourteen day. In the second month, a decrease in the levels of reactive C-protein was observed, as well as D-dimer and neutrophils and an increase in the numbers of TCD3, TCD4 and NK lymphocyteswere observed. A decrease in lung extension was observedinthe fourth month. The improvement in all the parameters was maintained until the end of patient follow-up. Those data show that UC-MSCs can play an important role both in the early stages, by preventing moresevere complicationsand in the chronic phase, with a reduction in sequelae.
COVID-19 is a complex multifactorial disease that makes treatment difficult using a single strategy. The promising long-term safety and efficacy results shown in this trial indicate that UC-MSCs could be used as adjunctive therapy for critically ill COVID-19 patients. UC-MSCs showed beneficial effects for patient recovery in the shortterm through a decrease inCRS by secreting anti-inflammatory factors, reducing risk of thrombosis and, in the longterm, viareduction in kidney and pulmonary sequelae based on tissue repair. The combination of immunomodulatory therapy based on UC-MSCs and antiviral drugs could help accelerate patient recovery, attenuating disease progression.