PD-1 signaling in primary T cells

Abstract

Programmed death-1 (PD-1) is a cell surface molecule that regulates the adaptive immune response. Engagement of PD-1 by its ligands PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. While a great deal is known concerning the biologic roles PD-1 plays in regulating the primary immune response and in T-cell exhaustion, comparatively little is known regarding how PD-1 ligation alters signaling pathways. PD-1 ligation is known to inhibit membrane-proximal T-cell signaling events, while ligation of the related inhibitory molecule cytotoxic T-lymphocyte antigen-4 appears to target more downstream signaling pathways. A major obstacle to an in-depth understanding of PD-1 signaling is the lack of physiologic models in which to study signal transduction. This review focuses on: (i) signaling pathways altered by PD-1 ligation, (ii) factors recruited upon PD-1 phosphorylation, and (iii) exploring the hypothesis that PD-1 ligation induces distinct signals during various stages of immune-cell differentiation. Lastly, we describe models to dissect the function of the PD-1 cytoplasmic tail using primary cells in the absence of agonist antibodies.

Fig. 1. Schematic diagram showing the cytoplasmic tail of CD28 and Siglec family members

(A) Dimeric members of the CD28 family. (B) Monomeric and Siglec-like members of the CD28 family. (C) Members of the Siglec family. (D) Example of non-CD28, non Siglec member with a single ITIM and ITSM.

Fig. 2. CTLA-4 and PD-1 target distinct signaling molecules

Both PD-1 and CTLA-4 signaling inhibit Akt activation; however PD-1 ligation inhibits a more upstream membrane proximal step by blocking PI3K activation. In contrast, signaling by CTLA-4 preserves PI3K activity allowing expression of certain genes such as Bcl-xL, but inhibits Akt directly by activation of the phosphatase PP2A.

Fig. 3. Models to study PD-1 signaling transduction in primary T cells in the absence of antibodies

(A) T cells transduced with SL9-specific TCRs recognize and respond to base aAPC expressing HLA-A2 and HIV-1_GAG. (B) Introduction of PD-L1 to the aAPC diminishes antigen-specific responses. (C) Introduction of PD-1 by transduction permits the side by side study of T cells with varying levels of PD-1. (D) Use of labeled signaling molecules permits microscope-based localization and functional studies.