Nerdy Science Blog

I just got back my immunology essay today. I got distinction for this essay. I missed out some points, made a lot of grammatical mistakes, and also poor analysis and critical thoughts. I guess it’s better to copy and paste my assignments (after graded) as I don’t want to waste my time and efforts just to write it for the marker to read it once. Instead of keeping in my computer, I post it here for other students who might need a reference on this topic (please do not plagiarise).

Title: Cross-presentation of Antigens by MHC Class I Molecules

Introduction
Cross-presentation is important in immunological mechanism against tumours and viruses and it was first described by Bevan (Van Kaer, 2002; Bevan, 1976). It is the process of antigen-presenting cells (APCs), particularly dendritic cells (DCs), to capture extracellularly derived antigens (predominantly in the form of protein), process and cross-prime to CD8+ T cells (CTLs) via major histocompatibility complex (MHC) class I molecules (Guermonprez and Amigorena, 2005; Lizee et al., 2003; Shen and Rock, 2006). Many evidence showed that cross-presentation by APCs via MHC class I molecules were important to viral invasion. Many tests were conducted to understand the intracellular routes of cross-presentation in DCs. Further understanding of cross-presentation is also essential in designing better vaccine.

Importance of Cross-Presentation by MHC Class I molecules.
CTLs are important in antiviral immunity. The outcome of cross-presentation by MHC-I molecules can either be tolerization or priming of CTLs, which regulates and initiates immunity (Guermonprez and Amigorena, 2005; Fonteneau et al., 2003).

Most of the viruses do not have access to the routes for MHC-I cross-presentation to APCs due to the lack of specific virus receptor (Fonteneau et al., 2003; Freigang et al., 2003). This indicates that APCs present antigens acquired from other infected cells. APCs will then process these antigens in specialized pathway and load it onto MHC-I to cross-prime CTLs. Evidences showed that cross-presentations by DCs were the main mechanism that generated antiviral CTL-mediated immunity (Lizee et al., 2003). If there is any defection in cross-presentation elements, this can cause dysfunction of viral immune response. Mutagenesis in MHC class I cytoplasmic tail showed that transgenic mice could not respond to vesicular stomatitis virus (VSV) and Sendai virus (Lizee et al., 2003). Even if DCs were infected with vaccinia virus, they still can activate naïve CD8+ T cells in lymph node (Norbury et al., 2002). Cross-presentation does not only initiate antiviral immunity, but also important in maintenance of T cells tolerance to self-antigens (Fonteneau et al., 2003).

Intracellular Routes Involved In Cross-Presentation
Intracellular routes of cross-presentation had been studied extensively in DCs in vitro and in vivo. Figure 1 showed that there were five potential intracellular pathways being defined, which were vacuolar pathway, phagosome-to-cytosol-to-phagosome pathway, phagosome-to-cytosol pathway, GAP junction pathway, and retrograde pathway (Chen et al., 2008; Shen and Rock, 2006; Moron et al., 2004).

Figure 1. Potential mechanisms of cross-presentation (a) The vacuolar pathway. (b) Phagosome-to-cytosol-to-phagosome pathway. (c) Phagosome-to-cytosol pathway. (d) GAP junction pathway. (e) Retrograde pathway. (Shen and Rock, 2006)

Vacuolar pathway is transporter associated with antigen processing (TAP) independent and insensitive to proteasome inhibitors. Internalized antigens are degraded into peptides by cysteine protease cathepsin S in phagosome. Peptides are then loaded onto MHC-I. The resulting complexes are then transported to the cell surface through vesicular recycling (Chen et al., 2008; Shen and Rock, 2006). Cross-presentation in DC was blocked when protease inhibitor was used to inhibit cathepsin S activity in TAP-independent APC (Shen et al., 2004). Heat shock protein (HSP) enhanced murine macrophages which had their golgi inhibited by brefeldin A (BFA) showed that vacuolar pathway did not require ER and also TAP (Tobian et al., 2004). Inhibition of TAP in macrophages with US6 did not affect the rate of cross-presentation (Ackerman et al., 2005). Inhibition of proteasome did not block cross-presentation as well (Shen et al., 2004).

Cells have been known that they are electrically coupled through small channels. These small channels are called GAP junction, and they connect cytosols of adjacent cells. GAP junction allowed APCs to acquire peptides from other cells, and carried out cross-presentation. This pathway is called GAP junction pathway and it is TAP-dependant but proteasome-independent (Shen and Rock, 2006; Neijssen et al., 2005).

Phagosome-to-cytosol-to-phagosome pathway is proteasome- and TAP-dependent. MHC-I, TAP, Sec61 and other endoplasmic reticulum (ER) molecules are obtained by phagosomes by fusion with ER. Then the internalized antigen is exported to cytosol by Sec61 to the cytosol. The antigen will be degraded by proteasome and the peptides are transferred into phagosomes by TAP and bind with MHC-I (Shen and Rock, 2006). Sec61 also play a role in exporting internalized antigens to cytosol from ER and also introduce secretory or transmembrane polypeptides cotranslationally into ER (Ackerman and Cresswell, 2004).

Phagosome-to-cytosol pathway is proteasome- and TAP-dependent pathway. Exogenous antigens are internalized into phagosomes and transfer to cytosol. Proteasomes then degrade the antigens. The resulting peptides will be transport to MHC-I molecules in ER by TAP (Chen et al., 2008; Shen and Rock, 2006). Studies found that tyrosine-based endolysosomal targeting signal was important at sorting the MHC-I into vacuolar compartments (Lizee et al., 2003; Shen and Rock, 2006).

Retrograde pathway is proteasome- and TAP-dependent. Antigens are internalized into endosomes and retrograde into ER. Antigens will then be degraded by ER-associated proteasomal degradation (ERAD) in cytosol. Sec61 was found to involve in the retrotranslocation process (Ackerman et al., 2006). Its suggested functions were to move misfolded proteins into the cytosol for further degradation, and to remove unstable or accumulating proteins (Ackerman and Cresswell, 2004). Peptides are then transported back to ER and loaded onto MHC-I molecules by TAP (Chen et al., 2008; Shen and Rock, 2006). This model only cross-primes CD8+ T cells at high concentration in vitro and its importance is unclear (Shen and Rock, 2006). Endosome or macropinosome with ER membrane can also lead to wrong reasoning that ER is involved in this pathway (Ackerman et al., 2006).

Most of the routes also required proteasomes to degrade antigens to peptides. Proteasomes were found to degrade many antigens for easier loading onto MHC-I molecules (Fonteneau et al.¸2003). Different peptides are generated by different proteasomes (Ackerman and Cresswell, 2004). Tiwari et al. (2007) demonstrated that endosomal processing was important for generation of MHC-I binding peptides. Vacuolar proton pump was inhibited with pH neutralizing drugs with acidophilic amines, and the loading process of peptide to MHC-I was suppressed (Tiwari et al., 2007).

ER was essential to transport peptides loaded MHC-I molecules to the cell surface (Fonteneau et al., 2003; Brophy et al., 2007). When BFA was used to inhibit peptide transport through Golgi, the presentation of certain peptides on MHC-I was blocked, and TAP was not essential in presenting these peptides (Brophy et al., 2007). This indicated another potential intracellular route in cross-presentation. In contrast, Chen et al. (2008) demonstrated that the use of BFA had little effect on cross-presentation in immature DC and CpG-activated DC. This explained some described pathway, such as vacuolar and phagosome-to-cytosol-to-phagosome pathway did not use ER or Golgi for MHC-I presentation in certain APCs.

Up to 85% cross-presentation was blocked when TAP was inhibited by ICP47 expression in human DC (Fonteneau et al., 2003). TAP was needed in cross-presentation to transport antigens from cytosol into ER. Inhibition of TAP in DCs with US6 stopped the peptide loading on MHC-I molecules (Ackerman et al., 2005). There were some APCs still carried out cross-presentation when TAP was inhibited, and there were two possibilities for that which were some TAPs were not inhibited, or there were other routes that were TAP-independent. Cytoplasmic mutations in murine MHC class I cytoplasmic tail had no effect on fibroblasts transport of emerging MHC class I complexes, which migrated from ER through Golgi and to the cell surface (Lizee et al., 2003). Tapasin was not only also found important in bridging TAP and MHC-I molecules in mice, but was essential in stabilizing TAP (Garbi et al., 2003). Tapasin also found in TAP-deficient APCs but it was suggested that tapasin was not involved in fusion protein trafficking and processing (Tiwari et al., 2007).

Cross-Presentation and Vaccination
Knowing which cells present antigens, where does the cross-presentation occur in the body, and what antigens will be presented in certain routes help in better vaccine development. APCs’ molecular and cellular basis of cross-presentation need to be understood so that vaccine can be designed to delivered to cross-presentation APCs so that they can prime CTL.

Long cell surface lifetimes of MHC-I and delayed presentation of exogenous antigen can stimulate strong T cell response (Brophy et al., 2007). Vaccine can be designed to increase MHC-I lifetimes and delayed presentation of MHC-I. Factors that suppress cross-presentation can be studied and avoided in vaccination design. For example, increase adenosine (Ado) will suppress cross-presentation and consequently cause autoimmune response (Chen et al., 2008). The studies of how Ado levels affect CTL can be studies to optimize the effect of vaccines. Acidification affected endosomal functions in cross-presentation by human DC (Fonteneau et al.¸2003).

The cellular pH condition will also affect the cross-presentation by MHC-I molecules (Vermeulen et al., 2004). Inhibitions of vacuolar proton pump in mice also affect the endosomal activities (Tiwari et al., 2007). Vaccine can be designed to contain substances that can induce optimum pH condition in APCs so that activation of CTL can be improved.

As different antigens have different routes to process them to be presented in different APCs cross-presentation. Membrane-associated proteins were involved in TAP-independent pathway (Tiwari et al., 2007). Antigens used for vaccination could be enhanced to targeted specific APCs for better result. MHC-I molecules were found to be carriers of peptide epitopes, and vaccine can be manipulated to present suitable epitopes (Tiwari et al., 2007). Antigens can also be induced into other cells and present to APCs via gap junction (Neijssen et al., 2005). Vaccine can also be introduced to APCs via different methods that APCs take up antigens (Moron et al., 2004).

Proteasomes are also important in degrading antigens to be loaded on MHC-I. It was found that cathepsin D but now cathepsin B was responsible for cross-presentation enhancement in human DC (Fonteneau et al., 2003). This finding could also improve vaccination design which can be degraded by cathepsin D instead of cathepsin B. Understanding of HSP which can enhance antiviral and antibacterial immunity can also help to integrate HSPs into vaccine so that vaccine can be used to therapeutically generate stronger CTL responses (Tobian et al., 2004).

Conclusion
MHC-I molecules play a major role in cross-presentation. Cross-presentation is essential to activate CTLs in antiviral immunity. The intracellular routes of cross-presentation are still unclear. There are many things that can affect the result of experiment to define the routes, such as source of antigens, APCs used, and the source of organisms. There are many other cells other than DCs, can cross-present antigens to CTLs. More researches need to be done so that better vaccine can be developed to enhance CTL response.

References
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Ackerman, A.L., Giodini, A., and Cresswell, P. (2006), A Role for the Endoplasmic Reticulum Protein Retrotranslocation Machinery during Crosspresentation by Dendritic Cells, Immunity, vol. 25, pp. 607 – 617.

Ackerman, A.L., Kyritsis, C., Tampe, R., and Cresswell, P. (2005), Access of Soluble Antigens to the Endoplasmic Reticulum Can Explain Cross-Presentation by Dendritic Cells, Nature Immunology, vol. 6, no. 1, pp. 107 – 113.

Bevan, M.J. (1976), Cross-priming for a Secondary Cytotoxic Response to Minor H Antigens with H-2 Congenic Cells which do not Cross-React in the Cytotoxic Assay, Journal of Experimental Medicine, vol. 143, ppl. 1283 – 1288.

Brophy, S.E., Jones, L.L., Holler, P.D., and Kranz, D.M. (2007), Cellular Uptake Followed by Class I MHC Presentation of Some Exogenous Peptides Contributes to T Cell Stimulatory Capacity, Molecular Immunology, vol. 44, pp. 2184 – 2194.

Chen, L., Fredholm, B.B., Jondal, M. (2008), Adenosine, Through the A1 Receptor, Inhibits Vesicular MHC class I Cross-Presentation by Resting DC, Molecular Immunology, 2008, vol. 45, pp. 2247 – 2254.

Fonteneau, J.F., Kavanagh, D.G., Lirvall, M., Sanders, C., Cover, T.L., Bhardwaj, N. and Larsson, M. (2003), Characterization of the MHC Cass I Cross-Presentation Pathway for Cell-Associated Antigens by Human Dendritic Cells, Blood, vol. 102, pp. 4448 – 4455.

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Guermonprez, P. and Amigorena, S. (2005), Pathways for Antigen Cross Presentation, Springer Seminar of Immunology, vol. 26, pp. 257 – 271.

Lizee, G., Basha, G., Tiong, J., Julien, J.P., Tian, M., Biron, K.E., and Jefferies, W.A. (2003), Control of Dendritic Cell Cross-Presentation by the Major Histocompatibility Complex Cass I Cytoplasmic Domain, Nature Immunology, vol. 4, no. 11, pp. 1065 – 1073.

Moron, G., Dadaglio, G. and Leclerc, C. (2004), New Tools for Antigen Delivery to the MHC Class I Pathway, Trends in Immunology, vol. 25, no. 2., pp. 92 – 97.

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