Scientists have been increasingly aware that the invading pathogens are not entirely to blame for the severity of infectious processes since the excessive inflammation produced by the host plays a significant role.
In the present study, we evaluated the role of one polymorphism in the promoter of TNF-α gene. Our main finding was that the frequency of AG genotype and that of the AA+AG genotypes were significantly lower among CAP patients compared with controls and logistic regression analysis indicated a significant negative association of genotypes with CAP. The prevalence of “A” allele was also lower among patients, but the difference was not statistically significant.
Together, these findings could be construed as implying that TNF-α G>A promoter polymorphism confers protection from CAP development. Surprisingly, our findings came against the hypothesis that more TNF production by the “A” allele increases susceptibility to CAP. However, it is also reasonable to suggest a counter hypothesis: more TNF-α production ensures more complete pathogen eradication. This could explain the finding reported by a study of patients with influenza pandemic (H1N1) wherein the frequencies of “A” allele and GA genotype were significantly lower while those of the G allele and GG genotype were significantly higher among patients compared with controls . Similarly, another study reported that the patients with influenza-related pneumonia were more frequently homozygous for the G allele of the TNF 308 G/A polymorphism compared with controls .
Notwithstanding, one should be cautious while drawing firm conclusions from our current research due to the small sample size. Moreover, the inflammatory reaction requires the cooperation of many different genes. The small effect of each gene requires a large sample size to be elicited in association studies. Furthermore, −308 G>A polymorphism is not the only polymorphism inside TNF-α promoter; other polymorphisms include −1031 (T/C), −863 (C/A), −857 (C/A), −851 (C/T), −419 (G/C), −376 (G/A), −238 (G/A), −162 (G/A), and −49 (G/A) . Consequently, the level of TNF-α is likely the product of several of these polymorphisms and not just −308 G>A , necessitating a thorough evaluation of all polymorphisms in future larger studies.
It should be pointed out that, to the best of our knowledge, our current study is the first to evaluate the role of −308 G>A polymorphism in pediatric CAP patients, but a relevant study of 120 very low birth weight mechanically ventilated infants found that the incidence of nosocomial pneumonia was not significantly different between infants with GG genotype and those having AA/AG .
In another study of 69 adult patients with pneumococcal disease (61 with CAP, 5 with meningitis, and 3 having both), no significant difference in the distribution of TNF-α was found between patients and controls . Likewise, a large multicenter study found no significant difference in the distribution of alleles or genotypes of −308 G>A polymorphism between adult CAP patients and controls . Similarly, a meta-analysis of 12 studies (the great majority were adult studies) concluded that −308 G>A polymorphism (AA+AG vs GG) was not associated with CAP or hospital-acquired pneumonia (HAP) risk, but when subgroup analysis was performed, the polymorphism was found to be associated with pneumonia in Asians but not in Caucasians .
It seems that the latter multicenter study and meta-analysis somewhat discouraged researchers from pursuing further research on the issue. However, it should be noted that the situation in pediatric patients is far from clear and the influence of ethnicity needs a more thorough evaluation.
In addition to the effect of TNF-α polymorphism on CAP susceptibility, we evaluated a potential influence of the polymorphism on CAP severity. We found no significant association of alleles or genotypes with any indicators of CAP severity, including clinical severity scores, but we were not able to assess the influence of TNF-α polymorphism on mortality since none of our patients died. It is likely that the small sample size could be responsible for our failure to demonstrate an association of TNF-α with any indicators of CAP severity. However, previous studies generally point to lack of association of this polymorphism with CAP outcome.
Consistent with our findings, genotyping of 77 children with respiratory syncytial virus infection revealed no association of TNF-α −308 G>A polymorphism with any of the clinical outcomes, including severity scores of lower respiratory illness, oxygen saturation, lengths of oxygen supplementation, intensive care unit (ICU) and hospital stays, and the presence or absence of pneumonia and otitis media . However, another study of 277 Chinese adult patients with severe pneumonia-induced sepsis found that TNF-α “A” allele increased the risk of septic shock (OR = 4.28). Moreover, the combined GA+AA increased the risk of septic shock even after adjustment for confounding factors (OR = 2.96) However, no association of alleles or genotypes was found with mortality .
In a multicenter study, no significant association of allele or genotype of −308 G>A polymorphism with adult CAP severity and outcome was noted . Similarly, a meta-analysis concluded that genotype (AA+AG) was not associated with a higher risk of mortality from pneumonia compared with GG, but carriers of “A” allele had higher risk of having severe pneumonia .
The unexpected lack of association of TNF-α −308 G>A polymorphism with CAP outcome might be explained by the presence of additional factors which cooperate together to determine the level of inflammatory response which, in turn, is presumed to influence the final prognosis. These factors include the following: first, variations in the levels of pro-inflammatory cytokines other than TNF-α; second, variation in the level of TNF-α from the influence of other promoter polymorphisms; third, variations in the responses of different individuals to the same TNF-α level due to polymorphisms in TNF-α receptor genes or in the genes encoding signal transduction pathway molecules; fourth, the balance between pro-inflammatory and anti-inflammatory genes; fifth, the inflammatory response to CAP in children might be somewhat different from that in adults; and sixth, the pattern of cytokine response might vary according to the pathogen type since different organisms interact with different pattern recognition receptors (PRR), with different signaling pathways .
So, it should be born in mind that if the excessive inflammatory response is associated with severe CAP, it is not TNF-α alone which determines the level of this inflammation, and some patients may demonstrate excessive TNF-α gene expression but its effects become overwhelmed by the other factors. Undoubtedly, a large association study or a meta-analysis of a greater number of studies is needed for more clarification of the issue.
The main limitation of the present study is the small sample size. Another limitation is the lack of measurement of serum TNF-α level to evaluate its relation to different alleles and genotypes. In addition, patients with severe pneumonia were a minority among our patient cohort and it is possible that our findings could have been different if more patients had severe pneumonia. These limitations need to be avoided in future studies.