By Richard R. Watkins, MD, MS, FACP, FIDSA

Associate Professor of Internal Medicine, Northeast Ohio Medical University; Division of Infectious Diseases, Cleveland Clinic Akron General, Akron, OH

Dr. Watkins reports that he has received research support from Allergan.

SYNOPSIS: In the first study to investigate the potential interactions between bacterial infections and lymphatic function, researchers found that methicillin-resistant Staphylococcus aureus toxins killed muscle cells critical to the pumping of lymph fluid and led to prolonged lymphatic dysfunction months after the bacteria had been cleared.

SOURCE: Jones D, Meijer EFJ, Blatter C, et al. Methicillin-resistant Staphylococcus aureus causes sustained collecting lymphatic vessel dysfunction. Sci Transl Med 2018;10(424).

Skin and soft tissue infections (SSTIs) are a frequent occurrence in patients who have impaired lymphatic vessel function, such as chronic lymphedema. SSTIs also can lead to secondary lymphedema, thus serving as both a cause and effect of lymphatic dysfunction. Jones and colleagues investigated the effects of methicillin-resistant Staphylococcus aureus (MRSA) infections on lymphatic vessel function and the underlying molecular mechanisms involved.

Using a mouse hind limb model, the researchers found that infection with the USA300 strain of CA-MRSA increased lymphatic vessel diameter and led to weaker, less frequent contractions that were present up to 120 days after inoculation. Only 50% of the infected mice had observable lymph flow following MRSA clearance (35 days after inoculation) compared to 90% of mice in an uninfected control group. The number of lymphatic muscle cells (LMCs) surrounding the lymphatic vessels remained depleted as late as 260 days following MRSA infection. LMCs are responsible for pumping lymphatic fluid and, thus, are crucial for modulating an effective host immune response to an SSTI.

Next, the researchers cultured LMCs in vitro and incubated them with a MRSA-conditioned supernatant. After six hours, substantial numbers of LMCs were killed. This effect was found to be caused by toxins expressed in the supernatant. Because expression of many MRSA toxins is controlled by the accessory gene regulator (agr) operon, the researchers tested a mutant form of MRSA lacking agr against several types of cultured cells and in the mouse model. The agr-mutant MRSA did not produce the LMC-killing toxins. Moreover, lymphatic function, including the strength and frequency of vessel contraction, was significantly better in mice that were infected with the mutant strain than in those infected with a nonmutant strain. Other types of vascular cells also were killed by the MRSA toxins, including smooth muscle cells in the posterior tibial artery.

These data suggest that agr-dependent MRSA toxins cause long-term inhibition of lymphatic vessel function. Finally, nitric oxide is a vasodilator that causes lymphatic vessels to contract under certain conditions. When MRSA-derived lipoteichoic acid was injected into the mouse hind limb, lymphatic contraction decreased in a dose-dependent manner. Moreover, inducible nitric oxide synthase (iNOS) knockout mice infected with MRSA had a significant reduction in lymphatic contraction, and iNOS remained undetectable in hind limb tissue 60 days after the infection.

COMMENTARY

This study is the first to investigate the association between a specific bacterial pathogen, in this case MRSA, and lymphatic function. The researchers found a novel mechanism of lymphatic impairment: MRSA infection leads to the death of LMCs due to the activity of agr-dependent toxins. This can explain how the function of collecting lymphatic vessels can be impaired long after the MRSA infection has resolved and the toxins are no longer active. If true, then the combination of slow LMC regeneration post-infection with recurrent MRSA can account for the cycle of lymphatic deterioration and reinfection in some patients with SSTIs. Therefore, novel therapies that regenerate LMCs may be able to restore lymphatic function, thus reducing lymphedema and the risk for recurrent SSTIs.

Another interesting therapeutic strategy the investigators proposed is to block agr signaling using specific chemical inhibitors. Several of these inhibitors are in the developmental stages and represent an antibiotic-sparing option for SSTIs. Further efforts at targeting MRSA toxins, both in animals and humans, should be a research priority. Also, the role of nitric oxide in lymphatic vessel function during and after MRSA infection needs further clarification.

The study had a few limitations. First, the murine model is not a model of recurrent infection. Second, mice do not develop chronic lymphedema. Third, the authors did not investigate the effect of other common pathogens that cause SSTIs, such as Streptococci and methicillin-susceptible Staphylococcus aureus (MSSA), on lymphatic function. However, since agr-mediated toxin production occurs in MSSA (albeit in lower amounts), it seems reasonable to hypothesize that the effects of infections by MSSA on lymphatic vessels would be similar to MRSA.

Additional studies in humans are needed to test the mechanistic framework proposed by Jones et al that MRSA infection leads to impaired lymphatic function, and to identify the specific MRSA toxins that cause the death of LMCs. One hopes this will lead to therapies that break the cycle of SSTI, lymph flow deterioration, lymphedema, and recurring infections.