Although MP infection shows typically self-limited course even without antibiotics, macrolides are still recommended as the first-line therapeutics. However, a steadily increasing number of recent cases progress to refractory, severe, life-threatening MP pneumonia [1,2]. Extensive use and misuse of macrolides may cause the rapid emergence of macrolide resistance . Macrolides inhibit protein synthesis by binding to specific nucleotides of the 23S RNA in the 50S MP ribosomal subunit. Mutations at domain V of 23S RNA reduce the affinity of the macrolides for the ribosome, which develop macrolide resistance [6-9]. Since 2000, some studies have confirmed the increase of this microbiological problem throughout the world, the highest prevalence has been especially observed in East Asia. The published rate was reported to be 87.2% in Korea, 81.6% in Japan, and up to 90% in China [6,13]. In this study, we found that 92.39% of clinical MP strains harbored A2063G mutation from children with MPP. Notably, we didn’t detect mutations at positions 2064, 2067, or 2617. However, several other novel mutations were found, including a G to C transition at position 2601, a T insertion between positions 2589-2590 and 2612-2613, and a G insertion between positions 2586-2587. Further studies are required to test whether these new mutations contribute to macrolide resistance.
Previous studies focus on the minimum inhibitory concentrations of macrolides in vitro, and explore point mutations of MP clinical strains [10-12]. However, the clinical relevance of these mutations has not been clearly characterized and large sample studies is fewer. In this study, the clinical, laboratory and radiologic characteristics of MPP patients with mutant strains were similar with those patients without mutations. These results hint the clinical symptoms, laboratory and radiologic characteristics of MPP are generally similar between mutations and without mutations. Previous studies on the comparisons of clinical manifestations between the groups also reported no significant differences [14,16,17]. As we all know, refractory MPP is characterized by long duration of fever, and severe pulmonary inflammatory response. In this study, the prevalence of mutations in non-refractory and refractory MPP was 93.87% and 87.5% respectively, which suggested that the infection of mutant strains does not increase the refractoriness of MPP in children. In addition, some children infected with mutant strain were also cured by macrolides, may be on the account of anti-inflammatory property of macrolides. And the use of timely and effective immunomodulators is beneficial to improve prognosis [18,19]. Based on previous researches, the pathogenesis of MP consists of direct damage mechanisms, immune damage and inflammatory damage . While there is some value in investigating the clinical significance of genetic mutations in MP, it is probably necessary to consider other risk factors that may trigger refractory, severe, or life-threatening pneumonia, such as a more robust host immune response to inflammatory cytokines, interleukins(IL), alexin, CD4+ T cell and so on [21-23]. In refractory MPP, immunomodulators such as systemic corticosteroids or intravenous gamma immunoglobulin are considered to be an effective treatment option by reducing host inflammatory response [18,19]. Based on previous researches, MP infection enhances mucin production, neutrophil recruitment, and excretes inflammatory factors . Mucus cell hypersecretion, especially goblet cell hyperplasia, has been shown in airways of MP infected mice . Additionally, in children infected with MP, the levels of tumor necrosis factor-α (TNF-α), IL-1β, IL-6, IL-10, C1q, C3, C4 in serum increase to varying degrees [21,22]. Those cytokines, ILs and alexin may participate in some classical or bypass activation pathways, mediate inflammatory reaction and immune responses, and have various biological activities.
More interestingly, our study and Wang et al found that MP-DNA load at enrollment was significantly different between refractory pneumonia and non-refractory pneumonia groups , which may be a risk factor of refractory MPP because of direct damage caused by increased MP load. The direct damage mechanisms of MP infection include adhesion damage, destruction of membrane fusion, invasive damage, and toxic damage . These suggest that the higher and more persistent MP stimulation may induce a much stronger direct damage. To sum up, the occurrence of refractory MPP in children may largely depends on the interaction between MP and host immune response, regardless of mutations.
Our study is significant because it has compared the manifestations of MPP in children in a high macrolide resistance period for MP. But it also had several limitations. Firstly, our hospital is a tertiary hospital, the enrolled population may have included some very severe MPP cases and have a much longer period prior to hospitalization at our hospital than in previous studies. Secondarily, our sample size was relatively small, and there was no multicenter research. Lastly, minimum inhibitory concentration values were not measured.