Judah A Denburg, Susan D Denburg, Boris Sakic, Henry Szechtman
Lupus Research Group, Faculty of Health Sciences
McMaster University, Hamilton, Ontario, Canada
(adapted from an article in Lupus World 1:1,3, 1998)For many years our group has been studying the involvement of the nervous system in patients with lupus. By now it is well established that many patients with lupus have problems in both cognitive (related to thinking skills) and affective (having to do with emotional issues) areas. For example, patients with lupus may have difficulty remembering things they have studied or learned, coming up with the right word at the appropriate time, concentrating or attending to details, or figuring out directions. In addition, patients with lupus may suffer from mood swings, irritability, difficulties relating to spouses, or extreme emotional and mental fatigue, often without any obvious flareup of lupus itself.
We have shown that these difficulties in the cognitive and affective areas are related to the presence of certain autoantibodies directed against the nervous system and present in the blood or cerebrospinal fluid. However, we have not been able to prove that these autoantibodies, or for that matter any other factors, directly cause cognitive and affective problems. The difficulty has been mainly in studying the brain, since it is more complicated to study this tissue than, for instance, the kidney, which can be biopsied. Therefore, it has become helpful in recent years to turn our attention to animal models of lupus and see whether or not they develop nervous system problems similar to lupus patients, so that we can understand the mechanism of disease and develop treatment strategies in a more reasonable fashion.
Several different animal species develop lupus; principal among these are several different strains of mice that can develop many of the features of lupus spontaneously during the course of a life span. The NZB/W mouse develops lupus rather slowly over more than a year, characterized by involvement of the blood and kidneys predominantly with the presence of many typical autoantibodies such as antiDNA antibodies and ANA. These mice are also known to have some difficulties in learning, especially in tasks that require remembering to avoid something unpleasant such as a shock or other negative stimuli. However, the difficulty in studying NZB/W mice is the length of time it takes them to develop fullblown lupus.
Another animal model is the MRL mouse, to which we have turned our attention in the last few years. This mouse strain develops lupus quite rapidly from the age of 34 weeks to 16 weeks of age, with arthritis, hemolytic anaemia, kidney abnormalities, salivary gland enlargement as in Sjogren's syndrome, and many typical autoantibodies in the blood. The disease progresses rapidly in these animals and ends with a lifethreatening illness that resembles lymphoma.
Clearly, neither of these strains of mice (nor others which also develop lupus), is identical to human SLE. Human lupus is a mixture of several different factors, including genetic and environmental ones. However, in the animal models, the mice of a given strain all resemble each other almost identically, since they are inbred. Lupus mice can give us a picture of what can happen when lupus is "condensed" into one uniform disease: they fulfill almost all the criteria for lupus, rather than only several. Having said this, however, it is interesting that we can predict a lot of different abnormalities in human lupus from animal models and learn a great deal about their mechanisms and treatment. This has been done for kidney involvement and perhaps is very relevant to nervous system involvement as well.
We undertook several studies during the past six years to investigate whether or not MRL mice actually have problems in behaviour that resembles or can be partly related to human CNS lupus. We were extremely surprised to find that from the age of 34 weeks, at a time when autoantibodies are just forming but the disease itself has not yet developed fully (neither arthritis, nor kidney disease nor blood disorder), these mice behave differently than their littermates without lupus, although they are otherwise almost genetically identical. The typical abnormalities in behaviour seen in these mice are hesitancy to explore a new environment, touch new objects or sniff them, and decreased activity. Moreover, these mice cannot find a platform in a water maze that they wish to escape: in fact, these animals give up quickly and float in a "helpless" fashion in the pool of water if they cannot find the platform. This timidity staying at the edges of the cage and the "helpless" behaviour in the water are all the types of behaviour one sees in small animals such as mice when they are given repeated stress that they cannot avoid. This has been called "learned helplessness", and can usually be reversed by antidepressant drugs, meaning that it can represent a form of depression in these small animals. All this occurs spontaneously in MRL mice, without any external stress!
But that is not all. The MRL mice also develop abnormalities in their skills. It takes them longer to learn where the platform has been moved than their lupusfree littermates, and several other indicators suggest that they cannot remember things normally. Thus, the animals have both an affective and a cognitive abnormality in their behaviour compared to the same mice that do not develop lupus. What intrigued us was: what could cause this array of abnormalities? How could it be present so early in the course of disease? Why did this continue to get relatively worse with age?
To answer this, we needed to know a little more about the immune system brain abnormalities in lupus mice in general. Others had shown, and we also have found, that in the brains of lupus MRL mice, there are accumulations of white blood cells and a sign of inflammation. It looks as if in the MRL mouse brain an immune response is occurring, since some of the brain cells that normally keep the brain properly functioning are "activated", much like they are in the earliest phases of inflammation in, for example, the joints. Moreover, in MRL mice and some other strains, there are abnormalities in the structure of the brain itself which may or may not be related to behaviour problems. And finally, there are changes in the molecules that make the brain "tick" for example, in neurotransmitters, which enable communication between nerve cells, or in cytokines, which activate the immune system. Cytokines especially influence responses to stress and the ability to make the body's own corticosteroids. The corticosteroid hormonal system also called the hypothalamicpituitaryadrenal (HPA) axis is of vital importance in regulating normal behaviour, appetite, temperature, fatigue and response to stress. In MRL as well as other lupus mice, there appear to be abnormalities in the HPA axis and cytokines that may significantly affect behaviour.
How can we link the abnormalities seen in the brain with the abnormal behaviour we have observed? One of the ways of doing this is to recreate the same abnormal situation of MRL mice in the brain of normal mice, and see if the behaviours that are seen in MRL mice are also found in these altered normal mice. We had observed an increased level of the cytokine interleukin6 (IL6) in MRL mice. We, therefore, decided to increase the level of IL6 in normal mice or in the MRL control mice that do not normally develop behavioural abnormalities. What we found was astonishing: the higher we made the IL6 level in normal mice, the more likely they were to develop the abnormal behaviours, especially depressive ones similar to those we had seen in MRL lupus mice. Of particular interest was the development of "learned helplessness", as well as a lack of interest in drinking a sweet solution, which is also a marker of depressive behaviour. Thus, what spontaneously occurs in our MRL lupus mice could be recreated in normal mice simply by increasing, temporarily, the IL6 levels in the blood and cerebrospinal fluid. Since IL 6 is elevated in many inflammatory reactions, we have a clue now that one of the important causes of some abnormal behaviour in MRL mice is an increased level of IL6.
Another approach we took was to treat the MRL lupus mice with cyclophosphamide, an immunosuppressive drug used in human lupus to control kidney disease. When we gave this medication in doses that could control lupus in these mice, but at a point in time much before lupus had progressed throughout the body, we could reverse many of the depressivelike behaviours such as excessive floating in the water maze and the hesitancy to come into contact with novel objects. That is to say, simply giving cyclophosphamide, a drug that suppresses autoimmune responses, could reverse abnormal behaviour. In addition, those animals which became more normal in their behaviour after cyclophosphamide were the ones in whom both the inflammation (white blood cells in the brain) disappeared and the IL6 levels fell to normal. This suggested very strongly that an excessively active immune system was the reason for the behavioural abnormalities.
We have also been exploring the effects of antidepressant drugs on the abnormal behaviours in lupus mice, since some of these may mimic human SLErelated depression, an important clinical issue. While we do not have all the results to date, some of our findings suggest that certain antidepressants may be more effective than others in lupus mice; in fact, some of the MRL lupus mice are very sensitive to low doses of typical antidepressants and may actually get worse with them if we are not careful. Thus, the changes in the brain may be quite complex and require new kinds of medications which target both the immune and nervous systems simultaneously.
Knowing that the brain is inflamed in lupus, and that infiltration by cells and effects of cytokines can lead to both neurologic and "psychiatric" (i.e., behavioural) abnormalities, we are gaining confidence in using the animal models to plan the future of therapy of lupus with nervous system involvement. The literature on the brain in human lupus is very meagre, since autopsy findings are scarce and controlled studies are virtually nonexistent. Taking the animal models and constructing new approaches to controlling cytokine responses, altering inflammationcausing genes or manipulating the immune response via changes in stress and environment, may all help us understand the complex nature of the interaction between the immune system in autoimmune disease and the nervous system. What is beginning to emerge is that autoimmunity can affect the brain in a direct way, and that the brain in autoimmune animals, and possibly in people with SLE, is "turned on" in such a way as to lead to behavioural and other nervous system abnormalities. Thus, the key to changing and reversing cognitive and affective problems in patients with lupus is probably the same key that is required to turn off the autoimmune response in the organism as a whole.