Seventh Point: The Anatomy of a Best Guess
Obviously the body is designed to be healthy wherein the players and the equipment are supposed to come together and play a game for the benefit of the spectators. In MS, the players gain access to the stands and use their equipment to beat-up the fans. Why?
No one knows with certainly, but there are widely accepted guesses. Now my goal is to try and put all the above information into practice. Can I better understand the medical language of MS? Here’s my objective – to more fully comprehend a predominate hypothesis of how MS happens. Here’s the hypothesis (from the
UCSF MS Center ):
In this model, the process begins with an infection by a virus or other external antigen (a substance, foreign to the body, that excites an immune response). Once this antigen gets into the blood stream, it is "eaten" by a macrophage, which digests the proteins of the virus or antigen to form smaller particles called peptides. Some of these peptides are then brought to the macrophage cell surface, where they are displayed inside a hand-like structure called an MHC (major histocompatability complex) molecule. In the cartoon, the peptide appears as a small oval ball held in the MHC. This MHC-peptide display can be recognized by special receptors on the surface of a particular white blood cell called the T cell. Only the TH1 subset of T cells that have receptors with a perfect fit for the MHC-peptide complex will recognize this display. Once recognition occurs, the TH1 cell undergoes activation of its cellular functions. This activation results in a proliferation of the number of TH1 cells that are capable of recognizing the MHC-peptide complex and the expression of additional T cell surface receptors that are capable of sticking to the so-called endothelial cells which line the blood vessel wall. Once attached, the TH1 cell secretes chemicals called proteases that facilitate migration through the endothelial cells and into the CNS. Once the TH1 cell arrives in the CNS, it may encounter a glial cell. Like the macrophage in the blood stream, the glial cell is also capable of presenting a MHC-peptide complex to T-cells. In MS, there is reason to believe that the peptide presented by the glial cell is a breakdown product of myelin, which is indistinguishable from the original viral peptide recognized by the TH1 cell. This "mistaken identity” restimulates the TH1 cell to proliferate and undergo further activation of cellular function. This activation leads to the production of chemicals called cytokines, which are small proteins that have effects on other cells. Some of these cytokines produced by TH1 cells, such as interleukin-2 (IL-2), interferon-gamma (IFN-g), tumor necrosis factor-alpha (TNF-a), and interleukin-1 (IL-1), promote inflammation. Another subset of T-cells called TH2 cells secrete different cytokines such as interleukin-4 (IL-4), interleukin-10 (IL-10), and tumor growth factor-beta (TGF-b), which counteract or regulate pro-inflammatory cytokines. The balance between pro-inflammatory and immuno-modulating cytokines is probably important in regulating disease activity. An imbalance favoring pro-inflammatory cytokines may result in demyelination.
That was a LOT more detailed than the definition we started with. But some of it made sense, yes? I’d hope that more of it made sense than before you started reading this whole narrative. Let’s break it down into parts and try to understand it better.
In this model, the process begins with an infection by a virus or other external antigen (a substance, foreign to the body, that excites an immune response). Once this antigen gets into the blood stream, it is "eaten" by a macrophage, which digests the proteins of the virus or antigen to form smaller particles called peptides. Some of these peptides are then brought to the macrophage cell surface, where they are displayed inside a hand-like structure called an MHC (major histocompatability complex) molecule.
The 4 key terms in this part are Antigen, Macrophage, Peptides and Major Histocompatability Complex (in this case MHC Class 2). One of the structural problems with MS research is that no one really knows exactly what the cause is. In this case an unspecified foreign substance gets in the body, gets consumed by the macrophage and is broken down and some of the parts get made into peptides which can be displayed as part of the MHC Class 2 molecules on the macrophage surface. The next part says:
In the cartoon, the peptide appears as a small oval ball held in the MHC. This MHC-peptide display can be recognized by special receptors on the surface of a particular white blood cell called the T cell. Only the TH1 subset of T cells that have receptors with a perfect fit for the MHC-peptide complex will recognize this display. Once recognition occurs, the TH1 cell undergoes activation of its cellular functions.
I didn’t include the cartoon. But if you follow the link above, you can see it. A lot of the illustrations of peptides make the MHC Class 2 molecule out to look like a hotdog in a bun with the peptide being the hotdog. The white blood cell, T-cell, TH1 subset that is mentioned are all the same cell. See T Lymphocytes above if this is not clear. Here’s another way of seeing how these are grouped:
White Blood Cells
.....T Lymphocytes
..........CD4+ T Cells
...............T Helper 0 Cells
...............T Helper 1 Cells
...............T Helper 2 Cells
..........CD8+ T Cells
.....B Lymphocyes
.....Macrophages
So the T Helper 1 cell has the ability to connect up with the macrophage that is displaying the MHC Class 2 molecule. The T Helper 1 cell is stimulated by this display and wakes up and is activated.
This activation results in a proliferation of the number of TH1 cells that are capable of recognizing the MHC-peptide complex and the expression of additional T cell surface receptors that are capable of sticking to the so-called endothelial cells which line the blood vessel wall. Once attached, the TH1 cell secretes chemicals called proteases that facilitate migration through the endothelial cells and into the CNS.
Once activated the T Helper 1 cells multiply. The T Helper 1 cells also have the ability to attach to the lining of the veins that run through the brain. This lining is the endothelial cells. Ordinarily the T Helper 1 cells cannot penetrate the endothelial cells, but in MS it is believed that they do. Once this happens the blood brain barrier has been breached.
Once the TH1 cell arrives in the CNS, it may encounter a glial cell. Like the macrophage in the blood stream, the glial cell is also capable of presenting a MHC-peptide complex to T-cells. In MS, there is reason to believe that the peptide presented by the glial cell is a breakdown product of myelin, which is indistinguishable from the original viral peptide recognized by the TH1 cell. This "mistaken identity” restimulates the TH1 cell to proliferate and undergo further activation of cellular function.
We’re not sure about the glial cell, they are mentioned above regarding blood brain barrier and Oligadendrocytes. In this hypothesis the T Helper 1 cell encounters a glial cell which unfortunately has an MHC Class 2 molecule on its surface that contains a peptide made from myelin. This causes the T Helper 1 cell to recognize myelin as an antigen, meaning the T Helper 1 cell now thinks myelin is a foreign invader like bacteria. This causes the T Helper 1 cell to activate again and initiate a process of multiplying to “wipe out” the myelin.
This activation leads to the production of chemicals called cytokines, which are small proteins that have effects on other cells. Some of these cytokines produced by TH1 cells, such as interleukin-2 (IL-2), interferon-gamma (IFN-g), tumor necrosis factor-alpha (TNF-a), and interleukin-1 (IL-1), promote inflammation. Another subset of T-cells called TH2 cells secrete different cytokines such as interleukin-4 (IL-4), interleukin-10 (IL-10), and tumor growth factor-beta (TGF-b), which counteract or regulate pro-inflammatory cytokines. The balance between pro-inflammatory and immuno-modulating cytokines is probably important in regulating disease activity. An imbalance favoring pro-inflammatory cytokines may result in demyelination.
Cytokines are a huge area of interest in MS. In the example the T Helper 1 cell emits cytokines and these send the message to other cells that it’s important to be activated against myelin. I’ll need to take a closer look at Cytokines in a separate writing. The overall point here is that you can understand doctor speak.
There are other theories, this is not set in stone. Here’s
another idea about the cause of MS.