Molecular Biology and Immunology for Clinicians, 8 Pathogenesis of Autoimmunity–Apoptosis

Basic science for the clinician 42: handling the corpses: apoptosis, necrosis, nucleosomes and (quite possibly) the immunopathogenesis of SLE

Death happens. It is, in essence, part of life. Humans deal with death in a variety of different ways, but often by keeping it at arms' length. At the cellular level, there are many forms of death, part of the development of organs and tissues (apoptosis) and part of pathologic processes (necrosis). The former, as has been described in an earlier paper in this series, is designed to eliminate the corpse with no evidence that it was ever there. Clearance is usually swift and effective, avoiding inflammation and specific immune interventions or responses. However, there is gathering evidence that autoimmunity leading to systemic lupus erythematosus may be due to ineffective or improper clearance of apoptotic debris, making it proinflammatory and allowing it to become highly immunogenic. This formulation also suggests therapeutic options that have already been demonstrated effective in controlling models of human autoimmune disease. This article reviews some aspects of this theory and some of the molecular biologic features of necrosis, apoptosis, and other forms of cell death.

Basic science for the clinician 31: CD molecules of relevance to immunity, inflammation, and rheumatologic syndromes

Molecular biologic technology has allowed us to study some of the many proteins expressed by leukocytes at different stages of differentiation, activation, and proliferation. Being able to purify cells with 1 or more surface molecules (eg, by FACS or lysis) with monoclonal antibodies and complement and identifying changes as cells are stimulated or activated has given us real insights into what is happening and how we might be able to modify cells that are going astray (eg, malignancy, autoimmunity). Over the years, there has been remarkable cooperation between laboratories to bring order out of chaos; by trading reagents, scientists have been able to identify the molecules being identified by different laboratories and come up with standardized names, often within the CD, or "clusters of differentiation," framework. These names are not acronyms, and the function and role of the molecule bearing a certain CD designation is not apparent. Worse, the proliferation of numbered CD molecules (over 250, with more to come after another conference in 2004) makes interpretation of the literature very difficult for those not immersed in the field. Thus, I have chosen (nearly arbitrarily and often based on my own interests) a number of CD molecules to briefly describe, pointing out the clinical relevance of each. It is worth reflecting on the fact that many of these were discussed in previous contributions to this series (as is pointed out in the following article). For those of you with the fortitude to follow this series, see how far you (and all of science) have come!

Basic science for the clinician 37: Protecting against autoimmunity-tolerance: mechanisms of negative selection in the thymus

As noted in previous articles in this series, tolerance, the ability of the immune system to differentiate self from nonself and leave the former alone, is a vital characteristic of a successful (and safe) immune system. With the detection of the molecule called aire (autoimmune regulator), the mechanism whereby autoreactive thymocytes encounter extrathymic proteins within the thymus and therefore are deleted, is now far better understood; aire was the subject of a prior article in this series. The absence of aire leads to autoimmune polyendocrinopathy, proof that aire is the center of an amazing "filtering" system. However, there are other mechanisms at work. Irregularities in expression of other proteins such as hypoxia-induced factor-1 (HIF-1) and CTLA4, have been implicated in autoimmune disease, the former in rheumatoid arthritis, the latter in autoimmune thyroid disease and lupus. Defects in intracellular factors involved in transcription of key apoptotic proteins have also been implicated in the escape of autoreactive thymocytes from the thymus, leading to autoimmune and lymphoproliferative syndromes as well. Changes in the proteins that oversee acetylation of histone lead to differential patterns of gene expression. At least 2 proteins involved in this process, HDAC and nur77, have been implicated in changes in survival of thymocytes. Yet again, there are multiple layers at work in the immune system; I have no idea how many more will be brought to light, which are phylogenetically most ancient or which will prove the most clinically relevant. For now, it is enough to bask in our new-found knowledge and know that the time from laboratory oddity, to animal model development, to therapeutic and/or diagnostic applications grows shorter each year since the molecular biologic revolution.

Basic Science for the Clinician 27

Ancient protective mechanisms are in place, deep within our defenses against infection and malignancy, often unappreciated until homologous proteins found within less phylogenetically advanced organisms are identified. Such is the case with 2 major recent finds, the Toll-like receptors (TLRs) and nucleotide oligomerization domain (NOD) families of innate immunity molecules. These families of receptors have high specificity, limited heterogeneity, and no plasticity; nonetheless, they play a pivotal role in rapid initial defenses against pathogens. Moreover, studies of the mechanisms of TLRs and NODs show how they and IL-1 and IL-18 stand at the threshold of the adaptive immune response and help to accelerate specific immune responsivity. Nonspecific reactivity of these preprogrammed receptors may be how relatively nonpathogenic organisms like yersinia and chlamydia may drive the inflammation of reactive arthritis and atherosclerosis. The inflammation of rheumatoid arthritis may be magnified, if not initiated, by these innate mechanisms as well.