Corticosteroids Effects On Gastrointestinal And Immune System Health
Since the discovery of glucocorticoids in the s and the recognition of their anti-inflammatory effects, they have been amongst the most widely used and effective treatments to control inflammatory and autoimmune diseases. In recent years, a great deal of effort has been invested in identifying compounds that separate the beneficial anti-inflammatory effects from the adverse metabolic effects of glucocorticoids, with limited effect. It is clear that for these efforts to be effective, a greater understanding is required of the mechanisms by which glucocorticoids exert their anti-inflammatory and immunosuppressive actions.
Recent research is shedding new light on some of these mechanisms and has produced some surprising new findings. Some of these recent developments are reviewed here.
Natural and synthetic glucocorticoids remain at the forefront of anti-inflammatory and immunosuppressive therapies. They are widely used to treat both acute and chronic inflammations, including rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, psoriasis and eczema, as well as being used in treatment of certain leukaemias and in immunosuppressive regimes following organ transplant.
However, long-term use of oral glucocorticoids is associated with serious side effects, including osteoporosis, metabolic disease and increased risk of cardiovascular disease Wei et al.
Over the 60 years since the discovery of glucocorticoids, much has been learnt of the molecular mechanisms by which they act recently reviewed; Perretti and Ahluwalia , Necela and Cidlowski , Yeager et al. However, much also remains unknown and the past few years have provided crucial mechanistic insights into the dynamic nature of GR interactions with DNA and higher order chromatin structures, into the subtleties of ligand effects on GR function, the regulation of endogenous ligand access to GR, glucocorticoid effects on leukocyte differentiation and function and the mechanistic basis for some of the repressive actions of glucocorticoids, critical for their anti-inflammatory effects.
The host inflammatory response is a primary defence mechanism engaged immediately following injury or infection which is necessary to restore homeostasis following successful elimination of the injurious agent, ultimately leading to resolution and tissue repair.
Although categorically distinct, the innate the relatively non-specific immediate host defence system that provides a rapid reaction to infection and tissue damage and adaptive the more slowly acquired, highly antigen-specific response immune systems interact and often overlap during an inflammatory response. Indeed, although acute inflammation is largely mediated by the innate immune system, the adaptive immune system often plays a major role in chronic inflammatory disease, with dysregulated lymphocyte responses.
These cause vasodilation, increased capillary permeability humoral response and leukocyte emigration into injured tissues cellular response , resulting in the hallmark pain, heat, redness and swelling of inflammation as well as generating a chemotactic gradient to guide and activate recruited cells to the site of injury.
Although specific characteristics depend on the immune exposure e. Activated granulocytes, crucial to contain microbial infection, are rapidly attracted to the inflamed site, followed by monocyte emigration from blood vessels and subsequent maturation into macrophages. Once at the inflamed site, neutrophils undergo constitutive apoptosis, functionally isolating them from the inflammatory environment by loss of stimulated chemotaxis, phagocytosis, degranulation and respiratory burst Haslett et al.
Foreign antigens are taken up by antigen presenting cells; particularly dendritic cells, but also macrophages, that then migrate to draining lymph nodes where they instruct the adaptive immune system T and B lymphocytes , shaping the subsequent immune response. As the inflammatory response progresses and evolves, mononuclear cells predominate and resolution normally ensues. Successful resolution of acute inflammation is an active and highly regulated process and dependent on mechanisms engaged early in the inflammatory response that programme the trajectory and form of the subsequent resolution reviewed Savill et al.
Persistence of the initiating stimulus invariably leads to chronic inflammation, with the typical dysregulation between destructive inflammatory and excessive healing responses seen in diseases such as arthritis, atherosclerosis and asthma. Glucocorticoids inhibit many of the initial events in an inflammatory response.
They also promote the resolution of inflammation although the mechanisms by which they do so have received less attention than those associated with suppression of the initial response. Acutely, glucocorticoids inhibit the vasodilation and increased vascular permeability that occurs following inflammatory insult and they decrease leukocyte emigration into inflamed sites, effects that require new protein synthesis reviewed Perretti and Ahluwalia Most of the anti-inflammatory and immunosuppressive actions of glucocorticoids are attributable either directly or indirectly to the transcriptional effects of GR agonism which alters transcription of numerous genes in leukocytes, both up and down Ashwell et al.
It should also be noted that some anti-inflammatory effects of glucocorticoids are apparent within minutes and a number are independent of the transcriptional effects of GR see for example Croxtall et al. Many of these genes are commonly over-expressed during chronic non-resolving inflammation.
This was true in both the brain and the liver, raising the possibility that chronic hypothalamic—pituitary—adrenal HPA axis activation as may occur during chronic inflammation for example exaccerbates inflammation in the brain and elsewhere reviewed in Sorrells et al.
This contrasted with the metabolic actions of glucocorticoids which require gene activation by GR. This view has recently been revised, with the discovery that key anti-inflammatory actions of glucocorticoids are brought about through gene activation Clark, and see below. In contrast to gene activation by GR, which at the time was believed to occur exclusively through homodimers of GR binding to palindromic glucocorticoid response elements GREs Beato et al.
Considerable support for the transrepression hypothesis came from in vivo experiments in mice in which the wild-type GR was replaced by the dimerisation dim mutant GR dim mice Reichardt et al. Glucocorticoid-mediated repression of APdependent transcription and other pro-inflammatory genes was intact in these mice, in contrast to defective GRE-dependent transactivation Reichardt et al.
However, transactivation of some genes, including that encoding phenylethanolamine N-methyltrasferase PNMT , remained intact Reichardt et al. Thus, at least some, but not all, of the dimerisation-independent repressive actions of glucocorticoids may be wholly or partly dependent on induction of DUSP1.
The anti-inflammatory actions of glucocorticoid-induced genes have been recently reviewed Clark, GILZ knockout mice have not been reported, but AnxA1-deficient mice show defective glucocorticoid suppression of inflammation in carrageenin-induced oedema, zymosan-induced peritonitis and antigen-induced arthritis Hannon et al. ILdeficient mice develop autoimmune disease and chronic inflammation Kuhn et al. However, IL has been implicated in negative regulation of corticosterone synthesis, acting at the adrenal gland Koldzic-Zivanovic et al.
How they regulate IL and AnxA1 remains unclear. In vitro , glucocorticoids increase IL expression in monocytes Mozo et al.
Glucocorticoids increase AnxA1 expression on human monocytes and neutrophils in vivo Goulding et al. Transcriptional repression by GR has always been the subject of debate, as alluded to above, including the extent to which it is dependent or independent of direct GR DNA binding.
Subsequent work has confirmed this Reddy et al. In silico prediction, genome scanning, chemically directed sequence-specific disruption of GR binding and chromatin immunoprecipitation experiments have shown that sequences that match the GR consensus do not necessarily bind GR in cells Horie-Inoue et al.
Core GR binding sites vary considerably around the consensus Wang et al. Moreover, actual occupancy by GR is influenced by post-translational modification Blind and Garabedian, and depends on cell-specific factors So et al.
Thus, the context of the GR binding site is crucial with the outcome — repression, activation or even specificity MR vs GR — dependent on the cell-specific complement of transcription factors Grange et al. DNA binding induces conformational changes in the dimerisation interface that expose otherwise silent transcriptional activation surfaces van Tilborg et al. These conformational changes are exquisitely sensitive to the DNA sequence, with single base pair differences differentially affecting GR conformation and transcriptional regulation Meijsing et al.
These dynamic and gene-specific differences in chromatin remodelling by GR are likely to be highly cell-specific and could underlie the complex kinetics of glucocorticoid responses, where glucocorticoid responsive genes may exhibit alternate activation and repression, with poor correlation in some cases between GR binding to response elements and target gene response John et al. Elucidating the nature of GR interactions with target genes, especially in the immune system, will be crucial to understanding their anti-inflammatory effects, but the challenge will be to establish these actions in physiologically relevant settings.
GR is widely, almost ubiquitously, expressed. Thus, glucocorticoids affect virtually all immune cells and, moreover, precise effects depend upon differentiation and activation state of the cell McEwen et al. Nevertheless, several lines of mice with global alteration of GR have provided vital new information about the functions of GR both in regulating the HPA axis and immunity and inflammation.
In all these models the HPA axis is affected because of the central feedback actions of glucocorticoids, compensating for altered GR function in all but the hypomorphic and null mice. Recent experiments using conditional GR knockout mice as well as transgenic mice have also shed light on the cell-specific functions of GR during immune and inflammatory responses. Tissue-specific models reported include T cell-specific or myeloid cell knockout of GR as well as transgenic mice with thymus over- and under-expression of GR.
Some of the immune and inflammatory phenotypes of these various mice have been reviewed in detail elsewhere Herold et al. Glucocorticoid action in T cells and, in particular, in thymus, where naive T cells that have yet to encounter antigen develop, has been the subject of intensive research yet remains highly controversial. Double positive cells, the majority of the thymocyte population, are highly sensitive to glucocorticoid-induced apoptosis Purton et al.
Much in vitro evidence points to a crucial role for glucocorticoids in regulating T cell number, repertoire and function, yet the in vivo evidence is discordant.
This topic has been extensively reviewed Ashwell et al. Suffice it to say, complete lack of GR globally, in T cells alone or inability to dimerise GR dim mice does not appear to affect thymocyte number or subsets, although these thymocytes are completely glucocorticoid resistant Reichardt et al. Conversely, mice with a global increase in GR levels yGR mice or function GR ML mice have normal thymocyte numbers and subsets, yet both lines of mice show increased glucocorticoid sensitivity of thymocytes in vitro Reichardt et al.
However, transgenic mice with increased GR in T cells directed by the proximal Lck promoter or a doxycycline-inducible CD2 promoter show reduced thymic cellularity with increased thymocyte sensitivity to glucocorticoids in vitro Pazirandeh et al.
Finally, in 2 of 3 models in which GR density is reduced by expression of antisense GR either globally neurofilament promoter or in T cells Lck promoter , thymic cellularity is increased, albeit modestly Pepin et al. However, the 3rd antisense model also using the proximal Lck promoter showed the opposite effect, with reduced cellularity King et al.
Reconciling these discrepant results will no doubt inform on the cell-specific roles of GR in thymus and T cell selection. T cell GR is required to survive lethal activation of T cells. Interestingly, these mice showed much worse tissue damage in the gastrointestinal tract than control mice following T cell activation, although damage in other tissues was similar Brewer et al.
GR is also required in peripheral T cells for the immunosuppressive effects of glucocorticoid therapy in experimental autoimmune encephalomyelitis EAE; a mouse model of multiple sclerosis Wust et al.
In control mice, but not in GR LckCre mice, glucocorticoid therapy induced apoptosis of T cells in peripheral lymphoid organs and down-regulated adhesion molecules, thus reducing migration of T cells to the site of inflammation Wust et al. This suggests that GR density in peripheral T cells is a critical determinant of sensitivity and that despite the presence of functional GR, clinical glucocorticoid resistance can arise.
However, in sepsis, another model of inflammation, GR in T cells is not required for glucocorticoid suppression cited as unpublished data in Kleiman and Tuckermann The phenotype of mice with a conditional deletion of GR in myeloid cells has been reported by 2 groups Bhattacharyya et al. In both cases, the LysM promoter was used to drive Cre recombinase-mediated excision of a floxed GR gene.
This results in efficient excision in macrophages and granulocytes in GR LsyMCre mice, with variable excision in other myeloid cell types including dendritic cells and mast cells Clausen et al. Similar to adrenalectomised mice Bertini et al. In contrast to endotoxaemia, mice with deletion of GR in myeloid cells remained fully sensitive to therapeutic suppression of EAE by glucocorticoid, although they did show exacerbated disease Wust et al. GR in myeloid cells, but not T cells, are required for glucocorticoid suppression of contact allergy a T cell-dependent delayed-type hypersensitivity response, such as occurs in response to metals or poison ivy Tuckermann et al.
Interestingly, in contrast to irritant-induced skin inflammation Reichardt et al. Thus, the immunosuppressive effects of GR are likely to result from multiple mechanisms, which are cell-type and stimulus-type dependent. The elucidation of the cell-specific roles of GR within other leukocyte populations and also within T cell subsets will be informed by future conditional knockouts.
Knockout of GR in mast cells, a major target in glucocorticoid suppression of allergic responses Kassel and Cato, , will be revealing, as will GR disruption in B cells. Like T cells, glucocorticoids also reduce circulating B cell numbers. Elucidation of the underlying mechanisms may be helpful in the treatment of some early T and B cell leukaemias that respond to glucocorticoids. As well as profoundly affecting the function of immune cells, glucocorticoids also alter differentiation programmes of progenitor cells McEwen et al.
Thus, chronically stressful conditions when endogenous glucocorticoid production is high or glucocorticoid pharmacotherapy may alter immune cell differentiation and indeed, probably shape the immune response as it develops Munck et al. This may be of relevance in the induction of peripheral tolerance to allergenic stimuli, a major clinical application of glucocorticoids. Dendritic cells are key antigen presenting cells that bridge the innate and adaptive immune systems.
Immature dendritic cells are activated when they capture, process, then present antigens, maturing into immunostimulatory cells in the process.
Activated dendritic cells migrate to draining lymph nodes where they interact with naive T cells to instruct the adaptive immune response.
Suppression of dendritic cells maturation and function has been implicated in the immunosuppressive effects of glucocorticoids. Nevertheless, the active suppression of antigen-specific immunity through T reg induction by glucocorticoid-programmed dendritic cells undoubtedly contributes to their efficacy in allergic disease and probably other chronic inflammatory diseases.
Interestingly, as well as promoting a tolerogenic phenotype in dendritic cells, glucocorticoids may also contribute to tolerance through direct effects on T cells. A more complete understanding of these effects will improve clinical use of glucocorticoids.
These dramatic effects of glucocorticoids on haematopoietic cell differentiation are not restricted to dendritic cells. Glucocorticoids induce a similar anti-inflammatory phenotype in macrophage differentiation.