A Taiwanese study has demonstrated that human placental mesenchymal stem cells (PMSCs) were therapeutic in hypervirulent Klebsiella pneumonia (hvKp) intra-abdominal infections (IAIs) by secreting IL-1beta to augment neutrophil numbers and functions, while restraining immune responses by other leukocytes.

Treatment reduced the bacterial load throughout the peritoneal cavity, which can be sites of continued infection, improving patient survival and indicating a therapeutic role for PMSCs in hvKP infections, the authors reported in the September 29, 2020, edition of Cell Reports.

"This is not only the first study to show that PMSCs are therapeutic for hvKP-IAI, but also the first to demonstrate the clinical relevance of human MSCs in treating the hvKP strain," said study leader Betty Linju Yen, deputy director of the Institute of System and Cellular Medicine at the National Health Research Institutes in Taipei.

Antibiotic-resistant bacteria represent a major healthcare challenge worldwide, with hvKP serotype K1 being the most prevalent isolate in severe IAIs, in both immunocompromised and healthy individuals.

The high infectivity and lethality of hvKP is due in part to a dense capsule layer enabling resistance to phagocytosis and evasion of polymorphonuclear neutrophils (PMNs), the first-line immune cells responding to infections.

While empirical antibiotic treatment has dramatically reduced IAI mortality, the emergence of multidrug-resistant (MDR) hvKP strains has resulted in high mortality rates in liver disease patients.

"Ceftriaxone is widely used against Gram-negatives such as K. pneumonia, which is also one of the most common species most prone to produce the extended spectrum beta-lactamase (ESBL) that confers drug resistance to cephalosporins such as ceftriaxone," said Yen.

Carbapenem is the most widely used alternative against ESBL-producing bacteria, but resistant K. pneumoniae strains producing carbapenemases have emerged, prompting an urgent search for alternative strategies.

MSCs

Besides their regenerative properties, MSCs have therapeutic potential due to their strong immunomodulatory effects. While these have been best delineated toward T cells, modulation to natural killer (NK) and myeloid-lineage cells have also been reported.

MSCs have been studied in bacterial infections, including K. pneumoniae, but MSC-PMN interactions during in vivo infection have not been assessed, so it is unclear whether MSCs can modulate PMN functions during infection.

Moreover, while PMNs mediate host defenses against pathogens, when responses are unchecked, as in complicated hvKP-IAIs, there are high rates of morbidity and mortality.

In the new Cell Reports study, Yen and her collaborators investigated whether MSC therapy could be viable for PMN-resistant hvKP, and the mechanisms involved in MSC-PMN interactions in such severe virulent infections.

The efficacy of MSC therapy for hvKP-IAI was initially evaluated for two sources of MSCs: bone marrow mesenchymal stem cells (BMMSCs) and PMSCs.

The researchers also established the first mouse model of KP-induced IAI using a strain of hvKP K1 isolated from a patient with liver abscess formation.

Both MSC sources were shown to preserve PMN viability in vitro, but only PMSCs significantly enhanced multiple PMN antibacterial functions, including phagocytosis, oxidative stress and PMN bacterial killing, which was shown to be due to PMSC-secreted IL-1beta.

"PMSCs were shown to enhance PMN antibacterial functions, including phagocytosis and reactive oxygen species (ROS) production, by approximately three-fold," Yen told BioWorld Science.

"We then used transcriptomic analyses to identify IL-1beta, a cytokine critical in activating numerous PMN functions, to be the PMSC-secreted factor modulating PMN functions," she said.

Moreover, in hvKP-infected IAI mice, PMSC treatment suppressed T-cell and NK responses while recruiting PMNs and enhancing antibacterial functions to confer survival against nearly full lethality at baseline.

"Our findings strongly implicate a possible therapeutic role for PMSCs towards severe, lethal hvKP infections, and highlight the importance of dissecting unique profiles of tissue-specific MSCs for improved effectiveness in clinical use," said Yen.

In addition, IL-1beta knockdown in PMSCs using short-hairpin RNA (shRNA) was shown to significantly reduce hvKP clearance, worsening survival and resulting in 100% lethality.

"We found that, compared with controls, shRNA IL1beta-knockdown-PMSCs reduced PMSC-beneficial effects on most PMN functions by 30-60%," Yen said.

However in vivo control PMSC treatment enabled slightly more than 20% survival compared to untreated infected mice with 5% survival. IL1beta-knockdown with shRNA "further reduced survival to 0% in the 100% lethality model," she explained.

"These data establish that PMSC-secretion of IL-1beta is critical in the preferential recruitment of PMNs during hvKP-IAI, resulting in enhancing bacterial clearance both locally within the liver and throughout the peritoneal cavity," said Yen.

"Importantly, these beneficial effects were shown to be achieved without eliciting excessive inflammatory damage and allowing for survival against absolute lethality," she added.

While much further work is needed to determine the safety and efficacy of PMSCs in humans, noted Yen, "we are currently working on how PMSC-secreting IL-1beta modulates other immune cells involve in both innate and adaptive immunity, during hvKP infection to further move towards clinical use," she concluded (Wang, L.-T. et al. Cell Rep 2020, 32(13): 108188).

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