Molecular Biology and Immunology for Clinicians 23

Basic science for the clinician 33: interleukins of current clinical relevance (part I)

Normal orchestration of wound healing, the protective immune response and inflammation, involve a bewildering array of cells that communicate to each other locally by means of cell-surface receptors and their ligands. For local and middle- to long-distance coordination, some of these same cells make and export soluble messengers that communicate to both immune and nonimmune cells. Although all these messengers have a role in normal immune homeostasis, it is apparent that many are involved in tissue damage in a variety of diseases, eg, rheumatoid arthritis and osteoarthritis. Some of these molecules are known as interleukins. The list of interleukins (IL) is now nearing 30 and, as a result of the molecular biology revolution, we now know the direct clinical relevance of many of them. As benchtop molecular biology matures into therapeutic and diagnostic tools, clinicians must begin to master this disparate group of molecules; making this harder is the fact that unlike acronyms (often impenetrable as they might be), an IL designation does not give a clue as to the source, target, or action of the IL. IL-1 and IL-2 were the first messengers to bear the "interleukin" name 25 years ago; they are well known to rheumatologists by now, so this article deals with some of the characteristics of the other clinically relevant IL molecules starting with IL-3.

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 38: B cells, factories, and immunomodulators

In a previous article in this series, we explored how developing pre-T cells learn how to be well-behaved T cells that recognize and honor the "self versus nonself" dichotomy of the universe. B cells do much of the same sort of thing, with multipotent stem cells becoming committed to ultimately becoming B cells within the bone marrow and then, after judicious culling of the flock, going off to the spleen to learn the final lessons needed to become "well-behaved" B cells. Like with T cells, there are a large number of things that can, and do, go wrong. If there is a failure of the system and B cells do not develop properly, hypogammaglobulinemia may develop as a result of a number of immune deficiency syndromes that can be quite devastating. If autoreactive cells survive to emerge into the periphery, autoimmunity, either organ-specific or more global, may occur. If B cells in the periphery proliferate in an uncontrolled fashion, a variety of B cell lymphoproliferative syndromes may develop, recognizable by the phenotypic markers of their originating B cell lineage level of differentiation. The full details of how autoreactive B cells survive and thrive, only to cause disease, are not yet clear, but identification of many of the phenotypic surface markers and circulating growth factors identified to this point have borne therapeutic fruit.

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 36: protecting against autoimmunity: tolerance and aire, the immunologic shadow, and other mechanisms of negative selection in the thymus

One of the miracles of immunology is tolerance, the ability of the immune system to differentiate self from nonself and leave the former alone. Autoreactive thymocytes (the cells that would otherwise differentiate into mature autoaggressive T cells) are deleted from the emerging immunologic repertoire within the thymus; only a very small proportion of thymocytes survive the thymus--perhaps 2% to 5% emerge to become mature T cells in the periphery, the rest dying to assure tolerance. This apparent wastage somehow works to the benefit of the developing animal and if the process works, all goes well for the maturing immune system. In some autoimmune syndromes, autoreactive thymocytes are not eliminated within the thymus by either clonal deletion or activation-induced cell death; these then emerge to wreak their havoc. However, how does this deletion happen? Recent research from a number of groups has (incredibly!) identified expression of nonthymic, nonlymphoid proteins within the thymus (casting of the "immunologic shadow of self"), and this is under the control of a single protein: AIRE (autoimmune regulator). By allowing autoreactive thymocytes to encounter a broad spectrum of proteins from extrathymic organs, aire allows deletion of these cells and the maintenance of tolerance. The absence of aire leads to autoimmune polyendocrinopathy, proof that aire is the center of an amazing "filtering" system. These and other molecular mechanisms underlying tolerance are explored in this and the next paper in this series.

Basic science for the clinician 32: T-cells with regulatory function

Tolerance is one of the major requirements of a successful immune system: destroy invaders but recognize self and the benign environment to leave them alone. Central tolerance is achieved by deletion of T-cells with T-cell antigen receptors that recognize self-antigens too well. However, within the population of T-cells that actually survive the harrowing experience of passage through the thymus (wherein over 98% of all pre-T-cells perish), there persist T-cells capable of inducing autoimmune damage. How then to avoid autoaggression? One theory in the 1970s was that there were peripheral T-suppressor cells that actively dampened these autoimmune proclivities. One problem presented itself, however; despite the fact that such an immunologic activity could be measured, no one could identify the cells that mediated the activity and so the concept fell into disfavor. However, the concept of peripheral regulation by a distinct (at last identifiable!) population of T-cells is now back in vogue. These T-regulatory (T-reg) cells have an important function in immune homeostasis. T-reg demand our attention as we try to manipulate the molecular biology of immune responses and inflammation to control autoimmune disorders and enhance transplantation efficiency.