Introduction: Monocytes are essential phagocytic cells of the innate immune system as they are required for
the maintenance of tissue homeostasis. However, accumulation of monocytes is implicated in various chronic
inflammatory diseases like coronary heart disease, atherosclerosis and in autoimmune disorders. Therefore, the
number of monocytes must be carefully regulated to avoid monocyte induced inflammatory disorders. Mesenchymal
stem cells (MSCs) have shown to be effective against various inflammatory diseases due to their immunosuppressive
properties. The present study was designed to evaluate the less understood immunomodulatory effect of MSCs on
monocyte proliferation and survival. Method: Primary monocytes were isolated from peripheral human blood using
CD14+ monocyte isolation kit. The in house produced umbilical cord MSCs were co-cultured with monocytes
at different ratio and time; assessed for the monocyte viability, proliferation and cell cycle. Results: Mesenchymal
stem cells suppressed monocyte proliferation in a dose-dependent manner. The antiproliferative effect of MSCs was
mediated by cell cycle arrest, whereby monocytes were arrested in the G0/G1 phase of the cell cycle by preventing
them from progress into S and G2/M phases. Although cell cycle arrest could potentially lead to apoptosis; however,
MSCs significantly enhanced the monocytes survival and inhibited apoptosis. Conclusion: Human MSCs inhibit the
stimulated monocyte proliferation without inducing cellular apoptosis at in vitro. These results reveal that MSCs can
be utilised to control monocytes’ quantity during an unwanted immune response to maintain homeostasis.