As the severe acute respiratory syndrome coronavirus (SARS-CoV)-2 is a new virus, the current knowledge on the immunopathogenesis of this newly emerged SARS-CoV-2 is beginning to unravel with intensive ongoing global research efforts. Although a plethora of new studies have been published in a short space of time describing how the virus causes disease and incurs insults on the host immune system and the underlying immunopathogenic mechanisms remain to be elucidated. Thus, the discussion in this review is based on the most current knowledge on the immunopathogenesis of SARS-CoV-2 that has emerged in the past 12 months. The main objective is to shed light on the most current concepts in immunopathological aspects of the lung, bloodstream, and brain caused by the SARS-CoV-2, which has led to the current pandemic resulting in > 100 million infections and > 2 million deaths, and ongoing.

Arginase-1 (Arg1) and the inducible nitric oxide synthase 2 (NOS2) compete for the common substrate L-arginine, semi-essential amino acid, and central intestinal metabolite. Both enzymes exhibit various, sometimes opposing effects on immune responses, tissue regeneration, or microbial growth and replication. In sub-mucosal tissues of patients suffering from inflammatory bowel disease (IBD), similar as in experimental colitis, the expression and activity of both enzymes, Arg1 and NOS2 are more prominent than in respective controls. Accordingly, the metabolism of L-arginine is altered in IBD patients. Thus, L-arginine represents a promising medical target for clinical intervention in these devastating diseases. Previous studies primarily focused on the host side of L-arginine metabolism. Initial reports using Arg1 inhibitors generated conflicting results in murine colitis models. Subsequently, only the generation of conditional Arg1 knockout mice allowed reliable functional analyses of Arg1 and the L-arginine metabolism in the immune system. Utilizing cell-specific conditional Arg1 knockouts, we have recently reported that Arg1, surprisingly, hampered the resolution of experimental colitis due to the restriction of the intraluminal availability of L-arginine. Reduced levels of L-arginine restrained the compositional diversity of the intestinal microbiota and subsequently the mutual metabolism between the microbiota and the host. Thus, the intraluminal microbiota represents a potential therapeutic target for L-arginine metabolism aside from host-dependent L-arginine consumption.

Skin is the largest organ of the body having multifunctional activities. It has a dynamic cellular network with unique immunologic properties to maintain defensive actions, photoprotection, immune response, inflammation, tolerogenic capacity, wound healing, etc. The immune cells of the skin exhibit distinct properties. They can synthesize active vitamin D [1,24(OH)₂D₃] and express vitamin D receptors. Any difficulties in the cutaneous immune system cause skin diseases (psoriasis, vitiligo, atopic dermatitis, skin carcinoma, and others). Vitamin D is an essential factor, exhibits immunomodulatory effects by regulating dendritic cells’ maturation, lymphocytes’ functions, and cytokine production. More specifically, vitamin D acts as an immune balancing agent, inhibits the exaggeration of immunostimulation. This vitamin suppresses T-helper 1 and T-helper 17 cell formation decreases inflammatory cytokines release and promotes the maturation of regulatory T cells and interleukin 10 secretion. The deficiency of this vitamin promotes the occurrence of immunoreactive disorders. Administration of vitamin D or its analogs is the therapeutic choice for the treatment of several skin diseases.

Mesenchymal stromal cells (MSCs) are a mesodermal stem cell population, with known self-renewal and multilineage differentiation properties. In the last century, MSCs have been widely used in regenerative medicine and tissue engineering approaches. MSCs initially were isolated from bone marrow aspirates, but currently have been identified in a great number of tissues of the human body. Besides their utilization in regenerative medicine, MSCs possess significant immunoregulatory/immunosuppressive properties, through interaction with the cells of innate and adaptive immunity. MSCs can exert their immunomodulatory properties with either cell-cell contact or via paracrine secretion of molecules, such as cytokines, growth factors and chemokines. Of particular importance, the MSCs’ immunomodulatory properties are explored as promising therapeutic strategies in immune-related disorders, such as autoimmune diseases, graft versus host disease, cancer. MSCs may also have an additional impact on coronavirus disease-19 (COVID-19), by attenuating the severe symptoms of this disorder. Nowadays, a great number of clinical trials, of MSC-mediated therapies are evaluated for their therapeutic potential. In this review, the current knowledge on cellular and molecular mechanisms involved in MSC-mediated immunomodulation were highlighted. Also, the most important aspects, regarding their potential application in immune-related diseases, will be highlighted. The broad application of MSCs has emerged their role as key immunomodulatory players, therefore their utilization in many disease situations is full of possibilities for future clinical treatment.