Psoriatic arthritis and the immune system

Printer-friendly version

Psoriatic arthritis is a disease intimately associated with the immune system. Many physicians are not aware that an inflammatory arthritis can be associated with psoriasis and thus obtaining a diagnosis often proves more difficult than it should be. Once diagnosed, however. There is a paucity of information that is available to either doctor or patient regarding this disease. Since its inception only a few years ago, the PAA has gone some considerable way in raising the awareness of PsA and contributing to the dissemination of information.

At the Medical College of St. Bartholomew's Hospital, the immunology and rheumatology departments have joined together in an attempt to better characterize the contribution of the immune system to the pathogenesis of PA. We are greatly indebted to the members of the PAA who have contributed their time and, in somecases, quite literally, a piece of themselves to our research. Results to date have been very interesting and will aid in understanding the way in which the immune system is involved in skin and joint inflammation caused by PsA.

During the PAA annual meeting there were many references to the search for a tissue type which is shared by people who suffer from PsA. The relevance of this information is not obvious and some readers may benefit from an explanation. The term “tissue type” refers to a collection of markers on the surface of the cells in our bodies. These markers contribute to distinguishing one person from another are used by the immune system as identity tags. The directions for the production of these markers are found in the genes of the major histocompatibility complex (MHC) which are passed from one generation to the next and are, therefore shared within families. At present, the search is on for a gene or collection of genes in the MHC which are associated with PsA. Other rheumatic diseases such as rheumatoid arthritis and ankylosing spondylitis occur more frequently in individuals with particular MHC genes.

It is thought that there is a genetic predisposition to the development of psoriasis and PsA but exactly which genes are involved is unknown at present. Evidence suggests that inflammation in the skin and joints is in part directed and maintained by cells of the immune system called T lymphocytes. Under normal circumstances these cells survey the bloodstream and body tissues for signs of infection and cancer. In the event that a foreign agent, such as a virus, is found, T lymphocytes destroy the infected cells and send messages into the local area to recruit assistance in clearing the infection.

For reasons unknown, T lymphocytes are present in large numbers in the psoriatic skin and inflamed joints of people with PsA. The activity of these T lymphocytes and the messages which they produce contribute to the thickened epidermis in psoriasis and damage in the joint. Many of the stronger drugs prescribed for PA such as methotrexate, azathioprine and steroids serve to reduce the production and/or activity of the cells of the Immune system, including T lymphocytes. It is through tissue type markers, the products of MHC genes, that T lymphocytes communicate with the rest of the body. These markers are required for a T lymphocyte to receive information as to the identity and state of healthiness of body tissues. If a particular tissue type were found to be linked to the development of PsA it would provide essential information regarding the seemingly inappropriate activity of T lymphocytes. It has been suggested for other diseases linked to certain tissue types that the affected tissue is altered such that the immune system mistakes it for something foreign. Alternatively, an unidentified infection may result in chronic inflammation either because it resides in inflamed tissue or causes confusion in immune cells resulting in healthy tissue being mistaken for infected tissue.

The big answers to the big questions like "What causes PsA and how can it be cured?" have yet to be found, but many doctors and researchers have begun to address essential smaller questions required for the understanding of PsA.

Since the seminal study of Moll and Wright identified PsA as a unique disease as opposed to the coincidental occurrence of psoriasis and arthritis, many groups around the world have attempted to characterize and discover better treatment for this disease. Advocacy groups such as the PAA are an essential resource for sufferers, medical professionals, the pharmaceutical industry and government. The PAA has facilitated the exchange of information between all these groups and increased the PsA awareness. Long may it continue.

Author: Elizabeth Ross,
Immunology Department,
Medical College of St Bartholomew’s Hospital,
London, UK.
First published 1997 Skin 'n' Bones Connection, Issue 8 p3-4