We walk a fine line between death due to immune deficiency, smothered under the weight of pathogens and parasites, and death by hyperimmunity, eaten alive by our own defenses. It’s amazing that our immune system can be tuned so precisely as to recognize anything foreign, yet ignore the vast antigenic universe of our own normal self.
Of course, sometimes the immune system fails, in both directions. We often hear about deaths from pathogens, and autoimmune diseases in general are pretty common. There are many ways by which (it’s believed) the immune system can become self-reactive, but a very common observation is that there are both genetic and environmental predisposing causes to autoimmunity. That is, you may have the genetic makeup to be autoimmune, but until you’re exposed to some environmental trigger, autoimmunity never develops. So, for example, if your identical twin has an autoimmune disease, you are much more likely than someone in the general population to develop the disease; but you still have a good to excellent chance of never getting the disease.
In many cases the neither the environmental triggers nor the genetic factors are well understood. The most likely environmental trigger, though, is some kind of microbe. In some cases, this may be because of “molecular mimicry” — the microbe has an antigen that looks like self antigen; the self antigen is normally ignored, because the immune system needs some kind of “danger” signal before it becomes activated; the microbial antigen is seen in the context of microbial “danger” signals; an immune response forms against the microbial antigen; the immune response cross-reacts with the self antigen; self cells are damaged by this immune response; the dead cells release more danger signals along with self antigen; and a positive feedback loop drives a full-fledged autoimmune disease.
That’s the model, but there aren’t many, if any, diseases where the whole process has been tracked through step by step; in fact, I think that there has been so much difficulty getting clear molecular connections between microbes and autoimmunity that there’s a robust search for other mechanisms. However, in the latest issue of Cell Host and Microbe, Albert Bendelac’s group shows a series of links between bacterial infection and the autoimmune disease human primary biliary cirrhosis (PBC).1 (There’s also a helpful, if rather dry, commentary2 by Sebastian Joyce and Luc van Kaer in the same issue.) Rather than trying to cover everything today I’m going to give background here, and then talk about the specific findings in a few days.
One interesting thing about Bendelac’s paper is that they link CD1 to the disease, through NKT cells. CD1 is an MHC class I family member; I talked about it back here, and that’s its mug shot to the left here (click for a larger version). CD1, like many members of the MHC class I family, has a “groove” in its “top” side. MHC class I proper binds peptides in that groove, but CD1 has a much more hydrophobic groove that binds to greasy things like lipids, glycolipids, and lipopeptides. These kinds of molecules are typically found in some kinds of bacteria — especially mycobacteria, like tuberculosis and leprosy, but also other kinds of bacteria such as the commensal microbe Sphingomonas.
MHC class I molecules, with their peptides, are recognized by cytotoxic T lymphocytes (CTL),3 but CD1 molecules and their lipids are recognized by a specialized subset of T cells, “natural killer-like” T cells (NKT cells). The function of this CD1/NKT system really isn’t all that clear. The early guesses that this was a branch of the immune system specialized for dealing with mycobacteria has been weakened as NKT cells have been linked to resistance to various viruses, and also as various viruses have been shown to block CD1 — suggesting that CD1 and NKT cells would otherwise eliminate them.
OK, enough for now. In my next post I’ll talk more about the disease itself, and then try to spell out the process by which, according to Bendelac, NKT are central to the autoimmune reaction; as well as how this abnormal reaction suggests some of the normal functions of NKT and CD1.
- Mattner J, Savage PB, Leung P, Oertelt SS, Wang V, Trivedi O, Scanlon ST, Pendem K, Teyton L, Hart J et al. (2008) Liver Autoimmunity Triggered by Microbial Activation of Natural Killer T Cells. Cell Host & Microbe 3:304-315.[↩]
- Joyce S, Van K, Luc (2008) Invariant Natural Killer T Cells Trigger Adaptive Lymphocytes to Churn Up Bile. Cell Host & Microbe 3:275-277.[↩]
- And natural killer cells, but let’s not go into that now[↩]