## 1 Microgravity in Human Health
### Microgravity and Immune Response  
A considerable body of investigations has demonstrated, in the past two decades, that microgravity has a significant impact on immune response, by acting on both innate and adaptive immunity, in a series of processes that can involve virtually all cells that participate in the inflammatory response (T lymphocytes, macrophages and endothelial cells), as well as the soluble molecular mediators released by them (such as cytokines, chemokines and, notably, bioactive lipids) [23; 24]. Since the 1980s, CD4+ T cells have represented a main target of investigation: seminal studies conducted between the 1980s and the 1990s (reviewed in [25]) demonstrated that authentic microgravity, or simulated weightlessness in a random positioning machine (RPM) or rotary cell cultivation system (RCCS), can profoundly depress activation and proliferation of concanavalin A (ConA)- and CD3/CD28-stimulated T lymphocytes. These findings were also confirmed by additional studies,  
Figure 1: Experimental setup of the main studies that investigated endogenous bioactive lipids in microgravity [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. RPM: random positioning machine; RCCS: rotary cell cultivation system. which demonstrated that immune suppression is accompanied by dysfunctional cytokine signalling. Indeed, peripheral blood mononuclear cells (PBMCs) isolated from astronauts, collected and stimulated just after post-flight landing on Earth, showed a significant reduction in CD4 T cell-derived interferon (INF)-\(\gamma\)[26], while purified PBMCs that had been exposed to Space displayed suppression of interleukin (IL)-2 production and down-regulation of IL-2 receptor CD25 [4; 27; 28; 5]. Remarkably, some alterations also became apparent under acute exposure to microgravity, and were recovered afterwards, as suggested by investigations showing that reduced INF-\(\gamma\) production from T cells was associated only with short duration flights (i.e., on the Space shuttle), while being absent in long-time missions (i.e., on board the ISS) [29; 30]. Moreover, system biology-based meta-analysis of published data has recently shown that microgravity acts on the different cells that are involved in inflammation, by acting on a core signature of roughly 100 conserved signalling pathways [31]. In particular, repression of the tumour necrosis factor (TNF)/NF-kB/Rel transduction axis, as well as of T-cell receptor (TCR)-mediated signals, was found in the transcriptomic, reactomic and miRNA array datasets of a number of studies [32; 33; 34; 35]. Moreover, the alteration of the adaptive immune response exerted by microgravity might have many implications, in that a recent work used a mouse in vivo model to demonstrate that Space flight impairs immune tolerance by significantly enhancing the production of inflammatory cytokines, such as INF-\(\gamma\) and IL-17, the latter molecule being engaged in the pathogenesis of a number of autoimmune conditions [36; 37]. This effect seemingly depends on the depletion of T regulatory (Treg) cells through the repression of the IL-2/CD25 axis [37]. Spaceflight can also act on the processes that orchestrate inflammation by targeting other cellular components of the immune responses, such as innate immune processes that orchestrate inflammation cells, as well as on vascular structures. Indeed, microgravity impairs macrophage differentiation and overall polarization [38] and alters complement system activation [39], while eliciting metabolic reprogramming and altering cytokine production [38; 40]. Recent studies have also reported a direct effect on endothelial cells, which consisted of the down-regulation of adhesion molecules that regulate the recruitment of immunocytes at the inflamed site [41], as well as of the enhancement of the Nlrp3-dependent inflammatory cascade [42].