Necrotizing Staphylococcal Pneumonia – The Tale of PVL

Staphylococcus aureus is an organism that is quite common in many diseases, including skin and soft tissue infections, bacteremias, and pneumonia. It causes fairly severe infection due to its significant number of toxins and virulence factors. I have mentioned the superantigen that causes toxic shock syndrome however another toxin that is useful to know about is the Panton-Valentine leucocidin.

PVL has been described as a pore-forming exotoxin that is formed by the assembly of 2 polypeptides (LukS-PV, LukF-PV; 1).  The existence of PVL was suggested way back in 1984, when Honore Van de Velde performed experiments in a model of pleural infection in rabbits and dogs (2). He inoculated the pleura of animals with 2 different strains of Staphylococcus aureus. In one, the WBC count increased in the pleural cavity while in the other, the leukocytes seemed to have been destroyed. Van de Velde described that the leukocytes had been “shot down. The cells were rounded, the nucleus condensed and the cytoplasm clarified with no granules and no pseudopodes; then the cytoplasm disappeared.” Further investigation by Philip Panton and Francis Valentine ultimately revealed that certain strains of S. aureus had leukotoxic activity on human leukocytes and little hemolytic activity. “Strains with strong leukocidin and weak hemolysin activity are commonly associated with acute severe infections, and the reverse combination is associated with long-standing superficial infection.” In other words, these toxins were able to destroy phagocytic leukocytes in human blood by forming pores in the cell and mitochondrial membranes, leading to lysis and apoptosis leading to liberation of inflammatory mediators. 

The most common involvement of PVL in staphylococcal infections are those of the skin and soft tissues (7), however I find the role of this toxin in pneumonia more interesting. It has been found to have a curious interplay with influenza infections, with a significant portion of those with necrotizing pneumonia having a preceeding flu-like illness prior to presenting, with most having severe presentations requiring intubation and mechanical ventilation (see later). As noted previously, PVL tend to exert is effect by “killing off” neutrophils, which in turn lead to the release of proteases and other cytokines that induce necrosis within the lungs. A group of investigators found that increasing doses of PVL lead to higher concentrations of IL-6, Il-8, and TNF-alpha (4):

Animals treated with rPVL exhibited neutrophilic infiltrate, necrosis, and diffuse alveolar hemorrhage along with pulmonary edema with four of the five animals tested displaying areas of necrosis, suggesting the role the toxin plays in the pathology. A more interesting study evaluate the interplay between viruses and PVL (5). It is hypothesized that viral infections activate epithelial cells by inducing a cytokine and chemokine response, which promote neutrophil migration into the inflammatory site. In this study,, the surface expression of CD11b was significantly higher when both influenza virus and PVL were incubated together, but not when they were incubated separately:

Furthermore, the incubation of both PVL and influenza virus lead to an enhanced rate of PMN death when compared to each one separately:

Infection of in vitro epithelial cells with either PVL-treated neutrophils or supernatant from influenza-infected cells lead to epithelial detachment, which was proportional to the amount of PVL used. Moreover, co-infection of Staph aureus and influenza lead to higher rates of PMN death, and in comparison with PVL-negative Staph aureus, SA USA300 (the one with +PVL genes) lead to higher rate of epithelial detachment:

Indeed, inoculation of mice with supernatant from PVL treated neutrophils revealed hemorrhages in different parts of the lungs compared to supernatant from untreated neutrophils, suggesting that PVL plays a role in necrotizing pneumonia. This means that influenza virus infection leads to a pro-inflammatory reaction in epithelial cells that involves the upregulation of chemokines that enhance the chemotaxis of neutrophils. During this period of time, lung tissue is intact (virus does not kill lung tissue). The action of PVL, which is cytotoxic to neutrophils, is thus enhanced by the preceding influenza infection. Low doses of PVL, thus, are able to play a more destructive role here. 

TL;DR

• Flu recruits bunch of neutrophils to the lung
• Small does of PVL kill off neutrophils in the lung
• Proteases kill off epithelial cells, and thus lungs become necrotic 

In an animal study (6), mice infected with strains isolated from PVL-positive staphylococcal pneumonia patients demonstrated symptoms of severe illness and mortality was higher when compared to mice infected PVL-negative isolates. Furthermore, increasing concentrations of LukS+LukF resulted in higher mortality, though inoculation of only one polypeptide did not result in mortality:

Clinical Picture:

The severity of this disease is highlighted in a case series describing 4 patients with no significant medical history (8). All presented with septic shock, and 2 of these ended up having preceding influenza infection:

Notably, those with PVL-positive Staph aureus pneumonia tend to be immunocompetent without significant risk factors for pulmonary infections and tended to do worse than PVL-negative patients. Moreover, many of these tended to be community-acquired infections, with one study finding that around 85% of pneumonia with community acquired staph aureus had PVL positivity (16).  

A cohort study (9) compared 16 cases of PVL-positive pneumonia with 36 cases of PVL-negative pneumonia. Those with PVL-positive pneumonia tended to be significantly younger (15 vs 70, p=0.001), were more likely to have a preceding influenza-like syndrome (12 vs 3, p <0.0001), and more likely to be hyperpyrexic. Also those in the PVL-positive group had no risk factors for infection, compared to most of the PVL-negative group. There were no significant differences in imaging patterns. Crude mortality rate was 75% in the PVL-positive group compared to 47% in the PVL-negative group; mortality at 48hrs was 63% in the PVL-positive group compared to 94%:

Presenting symptoms tended to be the same as that for pneumonia, with fever, dyspnea, and cough being the most common, however hemoptysis and multilobar involvement is significantly more common in these cases. For instance, a review of 92 cases (10) found that fever was the most common symptom (91%), followed by shortness of breath (68%), cough (63%) and hemoptysis (35%). Most patients tended to have multilobar infiltrates (78%). Mortality rate in this cohort was 43%, with 70% being admitted to the ICU. Logistic regression analysis found that leukopenia, flu-like symptoms, or lab-confirmed influenza, and hemoptysis were associated with increased mortality while therapy with either clindamycin or linezolid were associated with improved outcomes. 

Another review of 50 cases (11) of necrotizing staph aureus pneumonia found that only 20% of patients had a risk factor for respiratory disease, with 44% presenting with any type of hemorrhage and 78% having respiratory failure requiring intubation and mechanical ventilation. 67% of patients had a flu-like illness prior to the presentation. Mortality was 56% in this cohort, with initial presentation being severe as seen by the high median SAPS II value of 54. Mechanical ventilation, inotrope support, and ARDS were associated with higher mortality, with airway bleeding being associated with rapid mortality:

Erythroderma was also associated with increased mortality. Multivariate analysis found that leukopenia was associated with increased mortality:

Other interesting things to note is that those who died were also more likely to have lower platelet count (178k vs 92k) and lower P/F ratio (88 vs 53). 

Given the role of PVL, a toxin, in the pathogenesis, there is some suggestion that immunity to it would lead to improved outcomes. This was seen in 2 cohort studies. One cohort of 114 patients (12) with PVL-positive S. aureus pneumonia found that those with a prior furuncle (used as a marker for prior PVL-positive infection) had a significantly increased odds of survival within 30 days of hospital admission:

Univariate analysis of a review of 31 patients with PVL-MSSA pneumonia (13) found that influenza-like prodrome, absence of SSTI on admission, and septic shock were associated with death. Further, a multivariate model also found that influenza-like prodrome and absence of SSTI to be associated with mortality:

This would suggest that toxin-suppressing antibiotics, such as linezolid or clindamycin, may have a role in necrotizing Staph aureus pneumonia and in vitro data seems to suggest this. In one study (14), administration of oxacillin, vancomycin, clindamycin, and linezolid at concentrations above MIC suppressed the concentration of PVL. Sub-MIC concentration of clindamycin and linezolid lead to decreased concentration of PVL, however it increased with sub-MIC concentration of oxacillin:

Oxacillin also increased the beta-galactosidase activity, which suggest increased PVL transcription:

Another study (15) had similar findings, with sub-MIC concentration of oxacillin leading to increased PVL production:

Comparing combination of antibiotics, oxacillin and clindamycin seem to have the greatest effect on PVL production at any clindamycin concentration when compared to other concentrations:

While the increase in PVL concentration with sub-MIC beta-lactams is concerning, it should be noted there is little to any reason to actually use such low concentrations in clinical practice. This does, however, highlight the utility of clindamycin in necrotizing pneumonia as a toxin-inhibitory antibiotics similar to its use in invasive group A streptococcal infections. The role of IVIG has not been evaluated in the clinical arena, however the role of it has been seen in additional in vitro studies. In one study (17), the addition of IVIG to PMNs lead to the reduction of ethidium-bromide uptake (a marker of cell lysis; lower uptake, less lysis) when PVL or Staph aureus was added:

In another study (18), the addition of IVIG to either vancomycin or linezolid lead to higher survival in rabbits infected with PVL-positive staphylococcus aureus.

This suggests the role of IVIG as an adjunct, though how this translates clinically is unknown.

TL;DR

• Necrotizing pneumonia due to staph aureus is likely driven by the Panton-Valentine Luekocidin, which “makes a hole” in neutrophils and lyses them
• Preceding viral infection, usually influenza, leads to increased neutrophils in the alveoli that PVL lyses. This causes the release of proteases and necrosis of lung tissue
• Patients tend to be very young and very sick. Many have no risk factors and a significant percentage have a preceding viral illness
• Mortality and morbidity are high
• Therapy with beta-lactam or anti-MRSA agents and adjunctive toxin-inhibiting therapy, such as clindamycin, linezolid, or IVIG may have a role in improving mortality (moreso for the antibiotics rather than IVIG).

References:

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2. Lina, G., Vandenesch, F., & Etienne, J. (2006). A brief history of Staphylococcus aureus Panton Valentine leucocidin.
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