DC-mediated antigen-specific T cell activation

Once the antigen is processed and displayed on the DC cell surface, it is now available for specific recognition by naive T cells in lymphoid organs. In order for competent T cell activation to occur, three activation signals are required (Figure 1.4). The T cell must specifically recognise the peptide:MHC presented by the mDC via its distinct TCR (signal 1) (Murphy et al., 2012). CD8 and CD4 TCRs will specifically recognise peptides complexed to MHCI and MHCII molecules, respectively. Co-stimulatory receptor engagement between CD28 on T cells and CD80/86 on DCs comprises the second critical signal (signal 2) necessary for T cell

Figure 1.3 DC cross presentation pathways

Cross presentation of exogenous dying cell-associated antigens in conjunction with MHCI molecules to naïve CD8 T cells can proceed via three mechanisms (A) proteasome- and TAP dependent pathway (B) proteasome- and TAP-dependent ER fusion (C) protesome and TAP independent processing via the vacuolar post-golgi compartment. ER: endoplasmic reticulum; MHC: major histocompatibility complex; TAP: transporter associated with antigen processing.

21 Figure 1.4. Schematic diagram of DC-mediated T cell activation within peripheral lymphoid organs

Ag: antigen; IL-12: interleukin-12; mDC: mature DC; MHC: major histocompatibility complex; TCR: T cell receptor.

activation (Murphy et al., 2012). Co-stimulatory receptor signal transduction lowers the threshold for T cell activation, prevents T cell anergy, enhances T cell survival via up regulation of anti-apoptotic protein Bcl-xL and facilitates IL-2 cytokine

secretion (Bachmann et al., 1997; Okkenhaug et al., 2001; Acuto and Michel, 2003). It is important to note that as iDCs express low levels of co-stimulatory receptors, they present the antigen to T cells in the absence of signal 2, thus resulting in T cell tolerance (Kurts et al., 1998). Signal 3 represents cytokines secreted by the DC itself e.g. IL-12 or from other innate immune cells at the site of infection. This teriary signal is vital for the functional differentiation of CD4 and CD8 T cells into mature effector T cells. Once all signals are in place, this will result in the induction of competent antigen-specific naïve T cell activation. T cell activation proceeds via the triggering of complex tyrosine kinase cascades and other signalling pathways including p38 MAPK as reviewed in (Smith-Garvin et al., 2009). This results in the downstream activation and nuclear translocation of transcription factors Nuclear factor of activated T cells (NFAT), NF-κB and AP-1, which work in concert in the transcriptional upregulation of an array of genes responsible for T cell survival, proliferation, differentiation and cytokine secretion such as IL-2 (Macian et al., 2001; Hayden and Ghosh, 2011).

22 1.9 T cells and the immune response

Once a naïve T cell has recognised a specific antigen presented by the DC and is consequentially fully activated, it will undergo several rounds of proliferation. This results in its clonal expansion and functional differentiation into effector T cells. Naïve T cells are identified by their distinct cell surface expression of CD62L and chemokine receptors CCR7 and CXCR4 (Bromley et al., 2008; Kesarwani et al., 2012). Expression of CD62L, CXCR4 and CCR7 allow entry and residence in lymphoid tissues. Differentiation into effector cells is associated with down regulation of the aforementioned surface markers thus enabling them to leave the lymph node (Kesarwani et al., 2012). Migration to peripheral tissues and sites of infection through the blood stream and lymphatics is achieved through the upregulation of chemokine receptors such as CXCR3, CCR4, CCR5 and CCR6 (Bromley et al., 2008). Herein they perform specific adaptive immune responses upon re-exposure to the antigen and ultimately result in the clearance of infection and attainment of immunological memory. There are two major types of effector T cells, cytotoxic CD8 T cells (CTLs) and CD4 T helper cells. The type of T cell that is activated by an APC, depends on whether the pathogen was processed via the endogenous classical MHC I pathway or the exogenous MHC II pathway. CD8 CTLs and CD4 T cells are fundamental for providing defence against invading intracellular and extracellular pathogens, respectively. As stated previously, functional differentiation into competent effector T cells is highly dependent upon the cytokines present within their microenvironment (Figure 1.4, signal 3). For CD8 T cells, IL-12 acts as the third signal since its absence limits the capacity of the CD8 T cells to proliferate and to produce IFNγ (Curtsinger et al., 1999). Abrogation of this third signal also results in CD4 hyporesponsiveness (Tse et al., 2007). At the peak of the immune response, a minor population of cells diverge into memory T cells as typified by their expression of IL-7R (Zielinski et al., 2011). In contrast to naïve and effector cells, memory cells rely solely on cytokines IL-7 and IL-15 for their survival (Surh and Sprent, 2008). Therefore, once the infection and its associated antigen have cleared, the majority of T cells die en masse via various pro-apoptotic mechanisms within 7 days post- infection. The small population of memory cells which survive possess the

23 immunological memory of that specific antigen (Zimmerer et al., 2012). Unlike short-lived effector cells, antigen-experienced memory cells can survive for the lifetime of the human body providing long lasting immunity to that particular antigen (Zimmerer et al., 2012). Upon secondary encounter with the same antigen, memory cells undergo robust clonal expansion resulting in the rapid elimination of that specific pathogen (Zielinski et al., 2011).