Gut Inflammation in the Pathogenesis of Acquired Aplastic Anemia
Acquired aplastic anemia (AAA) is a severe autoimmune disorder characterized by the immune-mediated destruction of hematopoietic stem and progenitor cells (HSPCs). This condition arises from aberrant T-cell responses in genetically susceptible individuals, leading to the suppression and apoptosis of HSPCs. The pathophysiology of AAA is closely linked to systemic and local inflammation, driven by dysregulated autoimmunity. This dysregulation is marked by an increase in proinflammatory cells and a decrease in immunoregulatory cells. Notably, the upregulation of human leukocyte antigen (HLA) and Fas molecules on CD34+ HSPCs enhances antigen-presenting activities and accelerates apoptosis. The effectiveness of rapamycin in alleviating bone marrow suppression (BMS) further underscores the role of the abnormally activated mTOR pathway in this disease.
The current standard treatment for AAA is immunosuppressive therapy (IST), which typically involves a combination of anti-thymocyte globulin and cyclosporine A (CsA). While IST produces a response in approximately two-thirds of patients, the response rate is significantly lower in severe aplastic anemia (SAA). Patients who respond to IST often remain dependent on long-term CsA treatment. Despite its efficacy, the mechanisms underlying the initiation and perpetuation of chronic inflammation in AAA remain poorly understood. Various infectious and genotoxic agents have been implicated, and disease severity often fluctuates in parallel with physical and mental stressors, suggesting a role for chronic and recurrent infections. Recent studies propose that dysregulated autoimmunity in AAA may be driven by alterations in gut microbiota and compromised intestinal epithelium.
The human gastrointestinal (GI) tract is the largest and most vulnerable interface between the host’s psycho-neuro-endocrino-immune system and environmental exposures. It harbors the most complex microbial community and the most enriched gut-associated lymphatic tissue. Research in germ-free mice has demonstrated that the absence of gut microbiota leads to an underdeveloped immune system, highlighting the essential role of symbiotic microbes in immune system development and maturation. These microbes ensure appropriate responses to pathogens and tolerance to commensal microbes and self-antigens. The intricate communication between the intestinal epithelium, immune cells, and gut microbiota is critical for maintaining immune homeostasis and normal metabolism. In genetically predisposed individuals, disturbances in this balance can lead to chronic inflammation, impaired intestinal integrity, and increased mucosal permeability, fostering local and systemic autoimmune reactions.
Compromised intestinal epithelium allows intestinally derived antigens to translocate into the lamina propria and bloodstream, a phenomenon known as “leaky gut.” This increased exposure to exogenous antigens can trigger immune responses, leading to the activation of self-reactive cytotoxic T lymphocytes (CTLs). Th17 cells, activated by gut commensal microbes, can also provoke autoimmunity in remote organs. Changes in microbial composition and diversity, influenced by dietary protein sources, can alter mTOR activity and fuel inflammatory reactions. Reduced production of short-chain fatty acids (SCFAs) due to dysbiosis and insufficient indigestible polysaccharide supply can impair regulatory T cell (Treg) function, activate CTLs, and hinder epithelial repair, promoting type 1 immune responses. Dysbiosis and gut inflammatory conditions (GICs) may act as intensifiers, linking host immunogenetics with environmental challenges to amplify dysregulated autoimmune responses.
The association between GICs and AAA has been documented in several reports, including cases of inflammatory bowel disease (IBD), celiac disease, and neutropenic colitis. In many IBD-associated AAA cases, the development of AAA was attributed to the adverse effects of antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and immunosuppressants. However, successful treatment of drug-induced AAA with IST suggests a shared mechanism between AAA and IBD, as evidenced by the concomitant alleviation of GI symptoms. In some patients, AAA preceded drug use, indicating that these drugs may not directly damage HSPCs but rather disrupt the gut ecological environment or intestinal epithelium.
Celiac disease has also been associated with AAA, with concurrent diagnoses reported in several cases. Improvements in hematopoiesis and GI symptoms following a gluten-free diet highlight the pathogenic role of GICs in AAA development. In AAA patients complicated by neutropenic colitis, colitis was often attributed to severe neutropenia, though the presence of chronic gut inflammation preceding leukocytopenia suggests a more complex interplay. Similar to IBD-associated AAA, successful treatment with IST in these cases indicates an immune-mediated mechanism.
A case report of an SAA patient who responded unexpectedly to gut inflammation treatment further supports the role of GICs in AAA pathogenesis. This patient, refractory to standard treatments including CsA, stanozolol, rhG-CSF, and eltrombopag, experienced a prolonged febrile episode that resolved with gut infection treatment using mannitol and gentamycin. This treatment not only resolved the fever but also induced a significant hematological response. Subsequent recurrences were successfully managed with similar treatments, leading to prolonged hematological improvements. A preliminary investigation involving five other SAA patients and 27 non-SAA patients reproduced these therapeutic outcomes, further supporting the idea that GICs drive dysregulated autoimmunity in AAA. However, one patient developed refractory adynamic ileus and erythroid proliferative disease, ultimately succumbing to septic shock, highlighting the need for caution in identifying patients at risk of malignant proliferation after BMS resolution.
The precise mechanisms by which GICs contribute to AAA development remain unclear, but several potential pathways have been proposed. First, GICs may provide a continuous supply of exogenous antigens, facilitating the generation of self-reactive CTLs. In healthy individuals, the immune system returns to homeostasis after eliminating antigens, but in immune-compromised subjects, chronic BMS suggests a persistent antigenic challenge. Compromised intestinal epithelium allows high-dose exogenous antigens, cross-reactive with self-antigens on HSPCs, to interact with host immune cells, breaking immune homeostasis and activating low-affinity self-reactive CTLs. Endotoxins and microbial metabolites entering the bloodstream can trigger inflammation via pattern recognition receptors (PRRs) on antigen-presenting cells, perpetuating inflammatory cycles and suppressing hematopoiesis.
Second, GICs may skew the host immune system toward type 1 responses. Dysbiosis and reduced SCFA production lead to the skewed differentiation of CD4+Foxp3+ T cells into Th17 cells, decreasing the Treg/Th17 ratio and promoting a pro-inflammatory state. This overactivation of CTLs and overproduction of type 1 cytokines are characteristic of T-cell-mediated autoimmune diseases, including AAA.
Third, GICs may amplify pro-inflammatory reactions. Host immunogenetics shapes gut microbiota composition and abundance, influencing gut homeostasis and immune responses. In GICs, dysbiosis and compromised epithelium allow exogenous antigens to interact with host immune cells, priming inflammatory reactions via PRRs sensing pathogen-associated molecular patterns (PAMPs). This bidirectional interplay between host immunogenetics and gut microbiota can amplify autoimmune reactions in genetically predisposed individuals.
Finally, GICs may link host immunogenetics to environmental challenges. While immunogenetics is a major determinant of autoimmune disease susceptibility, environmental factors are indispensable. The GI tract, as the largest interface between the host and environment, bridges these factors. The increasing prevalence of autoimmune diseases, including AAA, in recent decades may be attributed to lifestyle and dietary changes.
Given the indispensable role of dysbiosis and GICs in immune-mediated pathophysiology, modulating gut microbiota and treating GICs may offer novel avenues for AAA research and treatment. However, several open questions remain. First, the role of BMS in limiting GICs, inhibiting pathogens that infect bone marrow and immune cells, and repressing malignant proliferation caused by genotoxic agents and intracellular viruses needs further evaluation. Second, the presence of low-dose lipopolysaccharide in blood and bone marrow, as well as PRR and HLA-DR expression on HSPCs in presenting exogenous and endogenous antigens, requires investigation. Third, the long-term effects of indiscriminate gut microbiota deletion on the host immune system and metabolism must be assessed. Fourth, genetic predisposition influencing AAA susceptibility and relapse needs to be evaluated and treated. Fifth, strategies to target memory CTLs to prevent rapid relapse should be developed. Sixth, the specific microbes and mechanisms driving AAA pathophysiology need to be identified. Finally, the optimal gut-modifying regimen for AAA treatment must be determined.
In conclusion, gut inflammation and dysbiosis play a critical role in the pathogenesis of acquired aplastic anemia. Understanding the interplay between gut microbiota, intestinal epithelium, and the immune system may provide new insights into the etiology and treatment of AAA. Addressing the open questions in this field will be essential for developing effective therapeutic strategies for this challenging disease.
doi.org/10.1097/CM9.0000000000000772
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