NASA has been organizing various space missions ever since the early 90s. Astronauts generally experience a non-gravity field during space flights. This condition is generally denoted as a zero gravity or more commonly, microgravity. Microgravity has been found out effect the basic signal transduction pathways of immune cells in human body. Though the exact mechanisms are still not known, there have been several reports that clearly indicate that the immune cells are greatly affected by microgravity conditions. Many investigators have reported these alterations caused by microgravity on the basis of results obtained from their studies. The first report about disturbed immune response was obtained in 70s, performed on the crew members/astronauts of Soyuz, Apollo and Skylab spaceships and it showed that the blood lymphocytes had exhibited reduced activity. Varicella zoster is a virus which stays latent after the primary infection. Though it has been found that it gets activated in organisms with a suppressed immune system, for instance, in patients having AIDS or cancer. The same virus V. zoster has also been reported to get sub clinically activated in astronauts. Also, it is well known that gravity can be perceived by special structures called gravireceptors that are, structurewise, statocyst-like organelles or gravisensitive ion channels present in the cell membrane. However, in unicellular organisms in which the sensitivity to gravity has been known to play an important role, the exact mechanisms of such processes concerning gravireceptors are not fully understood. These unicellular organisms mainly include paramecium and loxodes. In these organisms, gravisensitivity is also known to play a key role in determining cell behaviour. Much work in this regard is still in progress. Another revealing factor came into light almost 20 years ago, when Cogoli et al demonstrated that the proliferative response of T lymphocytes is greatly suppressed under the influence of microgravity. This generally occurs after the mitogenic stimulation.

Other significant studies in the field of microgravity reports the lowering in the proliferation activity of T lymphocytes. From the data obtained from the crew memners of STS shuttle mission, the expression of T lymphocytes appeared to be altered to a large extent. Most T lymphocyte cells were found to have a diminished response. Along with these alterations in T lymphocytes, huge alterations were observed in osteoblasts too. Osteoblasts are the cells responsible for bone development. Individual osteoblast cells were obtained from the crew members of Soyuz spaceship before and after the flight and then the results obtained were closely compared. The results indicated a low activity of osteoblasts which is obviously low if compared to the way osteoblasts behave in normal conditions. Moreover, the diminished response of T lymphocyte cells was found to be due to a reduced expression of Interleukin 2 receptor. The activity of lymphocytes (T cells) was also found to be reduced for successive production of cytokines under microgravity conditions. Thus, interesting enough, microgravity seemed to affect both physiological and chemical processes under microgravity's influence.

There has been several studies which indicate that the signal transduction pathways or to be clearer, the cell-to-cell communication is disturbed in microgravity. The data collected from the crew members and astronauts of Soyuz, Skylab and Apollo Spaceship say so.

Gravitational unloading: Changes observed in lymphocytes
Before getting onto the changes observed in T lymphocytes that are significantly attributed to gravitational unloading, a clear distinction must be made between two kinds of experiments: (i) In vitro experiments were performed with cells that were purified from the blood of test subjects obtained before flight and then exposed inflight to mitogens and other activators, and (ii) Ex vivo experiments performed with lymphocytes obtained from crew members of space missions exposed to mitogens before and after space flight. Significant results were obtained after the completion of the types of experiments. There was a difference in approach of the two, viz. first one was entirely based on basic cell biology while the second one was quite related to our topic here that is the effect of microgravity on signal transduction pathways of immune cells. The lymphocytes were centrifuged at 106 rpm and an alteration in their activity was observed that clearly showed that lymphocytes' activity gets reduced under microgravity conditions. Similar experiments were constructed on ground using clinostat like machines. These machines contained a rotating drum vessel that had an area where the cells were to be kept. On rotation of the vessel, it produced a condition similar to microgravity i.e. cells experienced free-fall. On the basis of these discoveries, conclusion can be drawn that the immune cell activation and proliferation is largely affected by microgravity.

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