T-cell Receptors and their importance in Organ Transplant and HIV-mediated Cell Death

Alex Guan, Henry Vo, Terence Chi Lok Cheung, Nicholas Loy Son Leslie Dan Faculty of Pharmacy, University of Toronto

1

PHM142 Fall 2014 Instructor: Dr. Jeffrey Henderson

Can be referred as T lymphocytes, part of the adaptive immune system2

From bone marrow -> develop receptors in thymus (gene rearrangement)3

Two main types: Helper T Cells and Cytotoxic T Cells

Cytotoxic T cells kills infected cells via toxins such as perforins and granzymes that cause the infected cell to undergo apoptosis1

Helper T cells secrete cytokines that activate cells involved in immune response such as macrophages (phagocytosis). They are also involved in the antibody production process of B cells1

What are T cells?

Heterodimer embedded in T cells2

Two polypeptide chains: α and β (more common) or γ and δ (less common)2

α polypeptide weighs 40-50kDa, and β weighs 40-45kDa2

Each polypeptide chain has a variable region and constant region (domains)3

Variable regions contain different sequences of amino acids which creates different binding sites for a variety of antigens3

T-cell Receptors

T-cell Receptor Structure

Dendritic cells phagocytize invading pathogens and use protease to break the proteins into peptides (antigens)3

This process is referred to as antigen processing3

peptides combine with MHC (major histocompatibility complex) and migrate to the surface of dendritic cells where they bind to specific T cell receptor3

Antigen-presenting cells3

How are receptors activated?

What Are MHCs?

Major Histocompatibility Complex

Cell surface molecules that bind to peptides from processed antigens4

Can act as antigen presenters to the corresponding T-cell receptors4

Two classes of MHC: 1 and 24

MHC Class 1 bind with intracellular antigenic peptides May signify that something is wrong with the cell itself (i.e.

tumours or bacteria/virally infected cells)4

These are found on all nucleated cells4

MHC Class 2 bind with extracellular-origin antigenic peptides These bind to peptides of pathogens to basically trigger an

immune response from certain immune cells by activating them4

These are found on very specific cells (such as B-cells or macrophages)4

Cytotoxic T cells (CD8+) are specific for MHC-14

Helper T cells (CD4+) are specific for MHC-24

MHC Binding Restriction

CD8 T-cells release cytokines such as IFN-γ and TNF-α to kill the target cell while CD4 T-cells release cytokines for activation4

In order to trigger a response, the MHC-peptide complex must4

Be able to bind perfectly to the TCR4

MHC Binding Restriction

T cells are infected by HIV in a receptor mediated fashion

Gp120 (on HIV) binds to CD4 (on the T cell), inducing a conformational change in gp120 allowing secondary interaction with co-receptor CCXR3 or CCR5.6

Subsequently, gp41 (on HIV) inserts into the plasma membrane of the T cell, undergoing a conformational change, ultimately fusing the HIV and T cell membranes together, allowing the viral contents to be injected into the cell. 6

Background: Receptor mediated entry of HIV into T helper cells

Image taken from http://upload.wikimedia.org/wikipedia/commons/c/c7/HIV_attachment.gif

When infected by HIV, there are two signature events which take place.5 Depletion of CD4 t-cell Inflammatory events

Human Immunodeficiency Virus (HIV)

Image taken http://labs.gladstone.ucsf.edu/greene/sites/default/files/imagecache/os_modal_image_500/greene/files/inflammation_cycle.png

Up until 2009, the mechanism of T cell death and the cause of inflammation was unclear.

Where does the Inflammation come from?

How is that related to cell death? Majority of cells die by a process called pyroptosis, a

Caspase-1 mediated response.5

Releases a large amount of inflammatory cytokines.5

Attraction of other T-cells “Vicious Cycle”

New drugs: Caspase-1 inhibitors

Mechanism of CD4 T-helper cell death by HIV-1 virus and possible new treatments

Organ Transplants: Allogeneic Grafting

In 2013, more than 4,500 Canadians needed an organ transplant7

Isogeneic Grafting Considerations ABO blood type compatibility8

TCR-MHC compatibility9

Low chances of compatibility, and high rates of rejection9

Hyperacute Rejection Acute Rejection Chronic Rejection

13Image taken from reference 10

Organ Transplants: Rejection Mechanism

T-Cell-mediated response to MHC 1 CD8+ cell TCRs recognize the foreign MHC 1on donor

cells9

CD8+ cells induce cytotoxic effects causing donor cells to undergo apoptosis

T-Cell-mediated response to MHC 2 CD4+ cell TCRs recognize foreign MHC 2 on donor cells9

CD4+ cells proliferate and secrete cytokines that activate macrophages

Antibody-mediated response to MHC 2 CD4+ cells communicate with B lymphocytes, which

create antibodies against MHC 2 and associated antigens9

14

R. CD8+

CD8

TCR

15

R. CD8+

CD8

TCRDonor APC

MHC 1

MH

C 2

T-Cell-mediated response to MHC 1

16

R. CD4

+

CD4

TCRDonor APCMHC 1

MH

C 2

Antibody and T-Cell-mediated response to MHC 2

Proliferation

R. CD4

+

CD4

TCR

R. CD4

+

CD4

TCR

Cytokine Production

IFN-γ

Activated Macrophages

B Lymphocytes

Humoral ImmuneResponse

17Image taken from reference 9

Summary T-cells are part of the adaptive immune system, The cells are produced in bone marrow and develop receptors in the thymus.

There exists helper T cells and cytotoxic T cells

The receptor is a heterodimer with two polypeptide chains, usually α and β although γ and δ exists as well The variable region provides different binding sites for the receptor to bind to different antigens

Dendritic cells break down pathogen to create antigens that bind to MHC in a process called antigen processing The MHC bound peptide is called antigen presenting cell and it then binds to T cell receptor MHCs are membrane molecules that act to bind antigen peptides in order to present them to T-cells for a variety of functions

2 classes: MHC-1/2; MHC-1 can be found on all cells, MHC-2 only on some CD8 binds to MHC-1, releases cytokines and triggers cell death CD4 binds to MHC-2, releases cytokines that trigger responses in a variety of different immune cells

In order for a T-cell to bind, the MHC and antigen peptide must be specific Even if one of the two doesn’t correspond, there will be no reaction

HIV depletes T-helper Cell (CD4+) populations and creates inflammation events HIV enters the cell in a receptor mediated fashion, using gp120 and gp41 glycoproteins to bind and insert viral contents,

respectively. CD4+ Cell death via pyproptosis releases pro-inflammatory cytokines, attracting other T cells which become infected,

propagating the infection as well as chronic inflammation. New drug prospect: Caspase-1 inhibitors.

It is important to consider both ABO blood type and MHC compatibility when discussing organ transplants. Incompatibility leads to either Hyperacute (preformed antibodies, within minutes), acute (days-months), or chronic rejection

(months-years). Rejection Mechanisms: T-Cell-mediated (CD8) response to MHC 1,T- Cell-mediated (CD4) response to MHC 2, and Antibody-

mediated response to MHC 2 + associated antigen Immunosuppression drugs + steroids can be used to treat rejection, however none target T-cell receptors

1. Kimball, J.W. (2011, April 5). Cytotoxic T Lymphocytes. Retrieved from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CTL.html

2. Owen, J.A., Punt J., Stranford S.A. (2013). Kuby Immunology. New York, NY: W.H. Freeman and Company.

3. Parham, P. (2009). The immune system. New York, NY: Garland Science, Taylor & Francis Group, LLC.

4. Murphy, K., Travers, P., Walport, M., & Janeway, C. (2012). Janeway's Immunobiology (8th ed.). New York: Garland Science.

5. Doitsh, G., Galloway, N. L., Geng, X., Yang, Z., Monroe, K. M., Zepeda, O., ... & Greene, W. C. (2014). Cell death by pyroptosis drives CD4 T-cell depletion in HIV-1 infection. Nature, 505(7484), 509-514.

6. Ray, N., & Doms, R. W. (2006). HIV-1 coreceptors and their inhibitors. In Chemokines and Viral Infection (pp. 97-120). Springer Berlin Heidelberg.

7. Government of Canada. (2013, December 13). Organ & tissue donation. Retrieved from http://www.healthycanadians.gc.ca/diseases-conditions-maladies-affections/donation-contribution-eng.ph

8. West, L. J., Karamlou, T., Dipchand, A. I., Pollock-Barziv, S. M., Coles, J. G., & McCrindle, B. W. (2006). Impact on outcomes after listing and transplantation, of a strategy to accept ABO blood group-incompatible donor hearts for neonates and infants. The Journal of Thoracic and Cardiovascular Surgery, 131(2), 455–461

9. Kumar, V., Abbas, A., Mitchell, R., & Fausto, N. (2007) Robbins Basic Pathology (8th ed.). Philadelphia: Elsevier - Health Sciences Division.

10. Welsh, R. M., & Selin, L. K. (2002). No one is naive: the significance of heterologous T-cell immunity. Nature Reviews Immunology 2(6), 417-426.

References