Mesenchymal stem cells in lung diseases and their potential use in COVID-19 ARDS: A systematized review

Highlights • None of the analized studies related serious adverse effects or toxicity to IV ASCs administration.• This review suggests optimism in IV ASCs for lung damage in severe COVID-19 ARDS.• Further studies on IV ASCs in COVID-19 are needed for standard dosage.


Introduction
The end of 2019 was marked by the growing number of cases of severe respiratory illnesses of unknown origin in Wuhan, China; in January 2020, its etiologic agent, the contagious Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) was identified [1]. Two months later, in March 2020, the World Health Organization (WHO) elevated a category of the 2019 Coronavirus Disease (COVID-19) from epidemic to the first pandemic caused by coronavirus, which on March 2, 2021 already illustrated a scenario with 2.6 million new confirmed and an increase of 63,000 deaths in the last week [2] 2 -Articles referenced by the works filtered from the search strategy that covered the eligibility criteria were also added.

Selection process according to the inclusion and exclusion criteria
Publications were selected using the search strategy previously described, without date or language limitation. Duplicates and titles not related to the topic were excluded before the screening.
The inclusion criteria choice was based on the PICO strategy. The study population included lung diseases, the intervention analyzed was the infusion of mesenchymal stem cells derived from adipose tissue, which was compared to conventional treatment or placebo saline infusion and analyzed for efficacy and safety.
In the first selection process abstracts were reviewed for the following inclusion criteria: (a) Administration of Intravenous (IV) ASCs, which (b) Were not used as a concurrent vehicle for other therapeutic agents, as (c) Treatment for acute or chronic lung diseases.
The second selection process excluded: a) Editorials, comments, and letters to the editor, in addition to articles that b) Discussed exclusively non-adipose stem cells and derivatives, or that c) Did not involve the intravenous administration of ASCs in d) Pulmonary immunoinflammatory diseases.

Endpoints
The evaluated outcomes can be divided according to two main approaches: efficacy and safety. The primary endpoint of the efficacy assessment was clinical parameters, while the primary endpoints of the safety assessment were descriptions of serious adverse events and death correlated to the intravenous administration of ASCs. Secondary outcomes included: a) For efficacyanalysis of the homing capacity of ASCs, serial imaging tests, histopathology, cytology, biochemistry, TUNEL method, PCRs, and immunohistochemistry, in addition to taking into account the study design, its participants, the origin of ASCs and dosage administered for comparative purposes; as well as b) Safetymild adverse effects (transient fever, diarrhea, bronchitis and common colds) secondary to the IV infusion of ASCs.

Results
After inserting the search strategy in databases, 2077 results were obtained, among which 1046 studies were initially excluded, then, based on the reading of titles and abstracts before the screening, only 231 articles were pre-selected ( Fig. 1). After evaluating the full text according to the eligibility criteria already described, 36 studies composed this review, being: 14 narrative reviews, 19 preclinical trials and three clinical trials. The clinical characteristics of these studies are summarized in Tables 1, 2 and 3. The search in the clinical trials database resulted in 29 studies of adipose-derived stem cells in lung diseases, their official status being: one no longer available, five unknown, five withdrawn, one enrolling by invitation, four recruiting, four not yet recruiting, one suspended, two terminated, six completed. No study has published its results in academic journals in the literature to date. The population, intervention, comparator and outcome of these studies are summarized in Table 4.
Searching the gray literature did not present results contemplated by the subject of the study.

Discussion
Although the mechanisms by which ASCs reduce lung inflammation and promote tissue repair are not fully elucidated [3], the use of mesenchymal stem cells in acute lung diseases had previously been reviewed by current literature showing promising results [13]. Since the initial analysis of the new disease caused by SARS-CoV-2 demonstrated main pathologic features similar to ALI/ARDS [14], the hypothesis of transposing these benefits in the context of a new pandemic without known therapeutic options were naturally investigated [1,3,14]. However, upon closer analysis, peculiarities were found in the pathophysiology of COVID-19 that benefited from autologous or allogeneic IV ASCs in a different way than those initially imagined [3].
In this context the present study proposed to analyze the benefits of cell therapy in COVID-19, exposing the possible common path among chronic and acute lung diseases that allow COVID-19 to manifest itself like chronic lung diseases [1,6], with fibrosis and pulmonary consolidation, but with an acute and fulminant evolution [6], owing to inflammatory exudation, pulmonary edema, and inflammatory cytokine storm.
Thus, the effectiveness evidenced by Liu et al. [3], Siu et al. [15]. and other studies is here revised as being due to immune dysregulation and fibrosis being common components of the pathophysiology of chronic and acute lung diseases, being closely related to their morbidity and mortality despite the different etiologies [7,13]. This convergence differs from a physiological immune response by inflammation resulting from both the activation of native pulmonary macrophages, molecular patterns associated with pathogens or associated damage, and the overproduction of alarmins that attract circulating immune cells to the lungs, initiating inflammation secondary to trauma and hypersensitivity [16,17].
Unlike similar studies that did not review the dosing regimen used, nor its effect on the studied endpoints, the present systematic review suggests that the fastest dose-dependent effect was exerted by cells cryopreserved at the primary site of infection [27] and the high dose showed not only a greater decrease in these parameters but also a low expression of αSMA and reversal of induced histopathological changes [26,43,44]. Therefore, and in accordance with other similar studies, this review suggests: the safety of IV ASCs [39,43−45] [31,39,43−45], based on the absence of serious adverse effects or toxicity to their administration, and the applicability of ASCs in ALIs of different pathophysiological mechanisms [5,6,14,20,23,28,29,31,37−39], including severe COVID-19 [1,6,26,40,43]. The physiological rationale reviewed suggests that therapy with ASCs can reduce lung damage in a patient with ARDS from SARS-CoV-2 infection, in addition to promoting leukocyte and lymphocyte recovery with its immunomodulatory and anti-apoptotic effects [12,17,26,40,43].  1. Flowchart of the selection process for researched articles. Legend: After inserting the search strategy in the databases, 2077 results were obtained, among which 1846 studies were initially excluded and only 231 articles were pre-selected, based on the reading of titles and abstracts. After evaluating the full text according to the eligibility criteria already described, 36 studies composed this review, being: 14 narrative reviews, 19 preclinical trials and three proofs of concept (N, Number).                  (continued on next page)  (continued on next page)  This study has among its limitations the selection bias, inherent to any non-systematic review; the limitation of most studies to interventions in the early inflammatory phase, offering better support for acute exacerbations to the detriment of its real applicability in the chronic fibrotic phase of the disease; the non-standardization of treatment time and dosage; as well as the lack of methodological rigor of some evidence included by not describing: their MSC surface markers, the parameters used in the analysis of the studies, nor the presence or absence of adverse effects.
Databases used in the present article are the main ones used in similar studies and allow contact with the vast amount of available literature on the subject. However, EMBASE database could not be included since CAPES periodicals does not provide its access through CAFe space. In addition, as it is a topic of recent emergence in the literature and, consequently, has an insufficient amount of clinical evidence for analysis, this study includes narrative reviews and preclinical studies to provide a summary of the currently available evidence on the topic, however, these study types have low-level certainty and high-level biases.
Finally, although the revised clinical data suggests optimism in the applicability of ASCs in other immunoinflammatory diseases [5,6,14 −17,20−23,28−31,37−43] the little clinical evidence available about the effectiveness of this treatment lacks standardization, making it difficult to extrapolate its results. Therefore, further studies are needed to be focused on the elaboration of a consensus on the methods of collection of ASCs, the ideal dosage schedule, the most effective time and route of administration, as well as on the definition of indications for the administration of ASCs in cases of COVID-19 for conducting clinical trials soon.

Conclusion
The revised clinical data suggests optimism in the applicability of ASCs in other immunoinflammatory diseases and in severe COVID-19 ARDS. However, further studies are needed to develop a consensus on the methods of collection of ASCs, the ideal dosage schedule, the most effective time and route of administration, as well as on the definition of indications for the administration of ASCs in cases of COVID-19 for conducting clinical trials in near future.