Our group has first established in past studies that independently of its cell cycle inhibitory capacity p21 is a negative regulator of immune activation and specifically T cell and macrophage function, and it might control other immune cell activity. Other groups confirmed our findings in a variety of models.  

A broader objective of our laboratory is to discover the mechanisms by which p21 attenuates T cells and macrophage activation and expand our studies in other forms of immunity and inflammation. Our actual main goal is to verify a relationship between p21 and mitochondrial ROS (mROS) which has emerged as a regulator of the immune response 

T cells mROS and p21 

 With regard to T cells, we have established that mROS controls the production of IFN-γ from previously activated T cells and we have verified the reduced production of IFN-γ through different mROS inhibition strategies. (Rackov et al. 2022)). Mitochondrial reactive oxygen is critical for IL-12/IL-18-induced IFN-γ production by CD4+ T cells and is regulated by Fas/FasL signaling.  

Our current work indicates that p21 modulates mROS and IFN-γ production by memory T cells, corroborating our published data and showing that p21 overexpression tempers autoreactive T cells and IFN-γ production. 

Macrophages mROS and p21 

In macrophages p21 modulates NF-κB activation but also allows polarization of M1 to M2 macrophages (Rackov et al. 2016). In order to study the mechanism by which p21 regulates macrophage activation through mROS, we first examined the kinetics of mROS production and its impact on the TLR4 activation pathway in stimulated macrophages. 

We have established that after activation of macrophages via LPS, early mROS signaling promoted the initial activation of NF-κB, MAPK and the production of proinflammatory cytokines, events that were reduced by inhibition or mROS extinction. Although it is generally believed that NF-κB activation drives mROS production, we verified that mROS promotes NF-κB activation. These mechanistic approaches are described in Figure 1.   

We discovered a role for p21 in the activation of mROS and in the response of these cells. We have verified a greater production of mROS by p21KO macrophages compared to wt macrophages, which leads to increased activity of NF-κB and production of cytokines. The proposed mechanism represents p21 as a primary regulator of mROS which controls of TRAF-6 ubiquitination and initiates NF-κB activity.   

Our future goals are directed in discovering the exact mechanisms that define the interactions between mROS, TRAF-6 ubiquitination and p21 in immunity, and design therapeutic approaches for inflammatory and autoimmune syndromes.