The Dual Power of Sulfasalazine in Pneumocystis Pneumonia Treatment

Pneumocystis jirovecii is an obligate fungal parasite that circulates throughout the human population. Although P. jirovecii does not produce noticeable disease in healthy individuals, it causes life-threatening pneumonia (PcP) in immunocompromised patients, such as HIV/AIDS patients, cancer patients undergoing chemotherapy, organ transplant recipients, or individuals on chronic immunosuppressive therapy. The mainstay of both treatment and prophylaxis for PcP is typically trimethoprim-sulfamethoxazole (bactrim), however, like all antibiotics, the misuse or overuse of bactrim can contribute to the development of antibiotic-resistance, which is a significant public health concern. Furthermore, the successful treatment of PcP is complicated by the need to not only eliminate the pathogen, but to also dampen the damaging consequences of the host’s inflammatory response.

Sulfasalazine is a medication primarily used to treat inflammatory bowel diseases, such as ulcerative colitis and Crohn’s disease. It belongs to a class of drugs known as disease-modifying antirheumatic drugs (DMARDs) and has anti-inflammatory and antibacterial properties. In addition to its use in inflammatory bowel diseases, sulfasalazine is sometimes used to treat rheumatoid arthritis, particularly in patients who have not responded well to other treatments. Previously, sulfasalazine has been studied for its effects on macrophages, particularly in the context of inflammatory diseases. Macrophages are a type of white blood cell that play an important role in the immune system. They are broadly categorized into two types: M1 and M2. The M1 macrophages are considered pro-inflammatory, produce inflammatory cytokines and are involved in the initial response to pathogens. In contrast, the M2 macrophages are anti-inflammatory and are involved in wound healing and tissue repair.  Sulfasalazine is known to have anti-inflammatory properties, and its effects on macrophages are part of this action. The drug can inhibit the activation of the NF-κB pathway, which is crucial for the production of pro-inflammatory cytokines by M1 macrophages. Moreover, by modulating the NF-κB pathway, sulfasalazine can reduce the release of pro-inflammatory cytokines like TNF-α, IL-1, and IL-6, which are produced by M1 macrophages. Furthermore, some studies suggested that sulfasalazine may influence the polarization of macrophages, potentially promoting the M2 phenotype over the M1 phenotype, although more research is needed in this area.

It’s important to note that the understanding of sulfasalazine’s effects on immune cells is still evolving, and much of the knowledge until recently comes from studies in the context of rheumatoid arthritis and inflammatory bowel disease. The exact mechanisms and the full extent of sulfasalazine’s impact on macrophages may vary depending on the pathological context. In a new study published in the journal Infection & Immunity, authored by Zhuo-Qian Zhang, Professor Francis Gigliotti, and Professor Terry Wright from the University of Rochester School of Medicine and Dentistry, the authors conducted a series of well-designed experiments to investigate the effects of sulfasalazine on PcP, particularly focusing on its interaction with macrophage polarization and the immune response. They found that sulfasalazine not only suppresses PcP-related immunopathogenesis, but also accelerates macrophage-mediated fungal clearance.

The team examined the effectiveness of sulfasalazine in the context of PcP using RAG2 knockout mice, including variants like RAG2/IFN-γR^−/− and RAG2/IL-4Rα^−/− mice that are impaired in their ability to generate conventional M1 and M2 macrophages, respectively. These RAG2 knockout models were chosen due to their lack of functional T and B cells, which renders them highly susceptible to Pneumocystis infection. The adoptive transfer of normal lymphocytes to the infected RAG2 knockout mice simulated a clinical presentation of PcP-related immune reconstitution inflammatory syndrome (PcP-IRIS). The PcP-IRIS mice were treated with either sulfasalazine or a placebo (phosphate-buffered saline, PBS), and the researchers monitored the severity of PcP and PcP-related Immunopathogenesis in these mice by measuring body weight loss, respiratory rate, lung compliance, and lung resistance. They found that sulfasalazine significantly reduced the physiological manifestations of PcP in the mice, evident by improved body weight, respiratory rates, and pulmonary function. They also evaluated the impact of sulfasalazine treatment on different lung cell types present in the bronchoalveolar lavage fluid, focusing on differences in populations like polymorphonuclear leukocytes (PMNs) and CD8⁺ T cells, which are associated with PcP-related lung injury. According to the authors, sulfasalazine treatment resulted in a notable reduction in the number of PMNs and CD8⁺ T cells in the lungs, suggesting a robust modulatory effect on the immune response during PcP. Moreover, imaging flow cytometry was used for quantifying macrophage phagocytosis of Pneumocystis, and the lung fungal burden in the mice was measured using quantitative real-time PCR and direct observation of Pneumocystis asci in lung homogenates. Sulfasalazine led to enhanced macrophage phagocytosis of Pneumocystis and a marked reduction in lung fungal burden in all mouse strains, indicating that sulfasalazine enhancement of fungal clearance did not depend on the classical M1 or M2 macrophage polarization pathways. Furthermore, the authors examined the polarization state of alveolar macrophages, particularly focusing on the expression of M2 markers like arginase. Despite the absence of IL-4Rα signaling in some mouse models, sulfasalazine treatment promoted M2 macrophage polarization, indicating an alternative pathway for macrophage activation. The study also suggested that sulfasalazine might exert its effects through mechanisms other than Th2 cytokine-mediated pathways, possibly involving NF-κB signaling, PPARγ activation, and adenosine release.

The implications of these findings are significant. They suggest that sulfasalazine could be an effective treatment for PcP, offering both antifungal and anti-inflammatory benefits. This is particularly important given the potential emergence of resistance to traditional treatments like Bactrim in Pneumocystis. Sulfasalazine could also provide a much-needed prophylaxis alternative, especially for patients who are already receiving it for underlying conditions like rheumatoid arthritis. In fact, a recent retrospective clinical study of rheumatoid arthritis patients lends support to these preclinical animal studies by demonstrating that sulfasalazine protects human patients against Pneumocystis infection (Nunokawa, 2023). In conclusion, the new study conducted by Zhuo-Qian Zhang, Francis Gigliotti, and Terry Wright offers valuable insights into the treatment of PcP, particularly in the context of immunocompromised patients. It showcases sulfasalazine potential as a dual-action drug that can both suppress the harmful immune response and enhance the body’s ability to clear the fungal infection, doing so through mechanisms beyond the conventional pathways of macrophage polarization.