PKC-u function at the immunological synapse: prospects for therapeutic targeting Alexandra Zanin-Zhorov 1 , Michael L. Dustin 1* and Bruce R. Blazar 2 1 Molecular Pathogenesis Program, Helen and Martin Kimmel Center for Biology and Medicine, Skirball Institute of Biomolecular Medicine, Department of Pathology, New York University School of Medicine, New York, NY 10016, USA 2 University of Minnesota Cancer Center and Department of Pediatrics, Division of Blood and Marrow Transplantation, Minneapolis, MN 55455, USA Protein kinase C (PKC)-u regulates conventional effector T (Teff) cell function. Since this initial finding, it has become clear that the role of PKC-u in T cells is complex. PKC-u plays a central role in Teff cell activation and survival, and negatively regulates stability of the immu- nological synapse (IS). Recent studies demonstrated that PKC-u is required for the development of natural CD4 + Foxp3 + regulatory T (Treg) cells, and mediates neg- ative regulation of Treg cell function. Here, we examine the role of PKC-u in the IS, evidence for its distinct localization in Treg cells and the therapeutic implica- tions of inhibiting PKC-u in Teff cells, to reduce effector function, and in Treg cells, to increase suppressor func- tion, for the prevention and treatment of autoimmune and alloimmune disease states. PKC-u location in the immunological synapse The protein kinase C (PKC) family of serine/threonine kinases contributes to signal transduction networks that co-ordinate almost all aspects of immune cell function and thus these kinases play a crucial role in immune regulation. There are three subfamilies of PKC, the conventional, which is activated by Ca 2+ and diacylglycerol, the novel, which is activated by diacylglycerol and the atypical, which is insen- sitive to both Ca 2+ and diacylglycerol [1] and all sub-families are represented in T cells. PKC-u is a novel PKC that is most abundant in hematopoietic cells [2]. PKC-u gained promi- nence with the finding that it is the only PKC isoform selectively recruited to the immunological synapse (IS) in conventional effector T (Teff) cells (Figure 1) [3]. The IS is a stable cell–cell junction that forms between a Teff cell and an antigen-presenting cell (APC) during TCR-peptide MHC recognition. The IS is composed of TCR microclusters that form the so-called central supramolecular activation cluster (cSMAC). This is surrounded by the peripheral supramolec- ular activation cluster (pSMAC), which contains large amounts of LFA-1 and ICAM-1 (Figure 1) [4–7]. Formation of the IS, which is crucial for sustained TCR-induced signal- ing and full T cell activation, leads to PKC-u recruitment to the junction between the cSMAC and pSMAC in a CD28 co- stimulatory-dependent manner [3,8–10]. Moreover, both CD28 and PKC-u are localized to a spatially unique com- partment that plays a central role in coordinating the signal transduction pathways downstream of the TCR [10,11]. PKC-u plays a major role in activation of NF-kB transcrip- tion factors and promotes AP-1 and NFAT signals [12,13]. Surprisingly, PKC-u negatively regulates IS stability by favoring symmetry breaking of the pSMAC in a manner counteracted by expression of the Wiscott Aldrich Syndrome protein (WASp) [14]. In this context, symmetry breaking refers to the conversion of the radially symmetrical pSMAC, which holds the cell in place, into an asymmetric lamella, which drives cell motility. The functional outcome of reduced IS stability might include a greater ability to gather antigen leading to prolonged signaling [15]. In addition, the sym- metrical pSMAC ring plays an important role in containing components that are released into the synapse. Consistent with this, inhibition of PKC-u can enhance killing by CD4 + cytotoxic T cells [16]. The detailed mechanism by which PKC-u stimulates symmetry breaking in Teff cells is not known, but in other systems symmetry breaking involves cooperative myosin II activation [17]. There are other mech- anisms for symmetry breaking; non-responsive anergic T cells, in which PKC-u is degraded, also break symmetry rapidly [18]. CD4 + FoxP3 + regulatory T (Treg) cells suppress inflam- matory responses in a TCR-dependent manner [19] and are central in maintaining a balance between immune toler- ance to self-antigens and anti-tumor responses [20–22]. There is evidence that stable Treg cell–dendritic cell (DC) interactions are important for Treg cell function in vivo [20–22], but the organization of the IS in Treg cells is not well studied. Recent data showed that during IS formation in Treg cells, PKC-u is sequestered away from the IS to the distal pole complex (Figure 1) [23]. Thus, PKC-u is differ- entially localized in Teff cells and Treg cells during IS formation. Furthermore, through use of supported planar bilayers to present anti-CD3 antibodies and ICAM-1 to T cells to mimic IS formation in vitro, Treg cells were found to form a more stable IS than Teff cells. Here, we examine the function of PKC-u in Teff cells and Treg cells with emphasis on the role of PKC-u at the IS. Also discussed is the therapeutic potential for targeting PKC-u as an anti- inflammatory approach. Review Corresponding author: Blazar, B.R. (blaza001@umn.edu) * M.L.D. and B.R.B. contributed equally to this work. 358 1471-4906/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2011.04.007 Trends in Immunology, August 2011, Vol. 32, No. 8