Our prospective birth cohort study demonstrated no significant influence of maternal cigarette smoking on the percentages of lymphocyte subpopulations in newborns. This was surprising as we expected alterations of the proportions of lymphocytes in newborns of SM with higher replication rates of peripheral naive T cells, estimated by TRECs and RTL, which was thought to be caused by a pro-inflammatory potential of cigarette smoke components. However, with the exception of longer RTLs in newborns of SM, neither lymphocyte subpopulations, nor TRECs or cytokine concentrations were significantly different between newborns of SM and NSM.
Our findings were in contrast to a study showing significantly lower leukocyte counts with a most prominently decrease in segmented neutrophilic granulocytes, monocytes and lymphocytes in neonates of smoking mothers . In that study, number of smoking mothers was small (14 versus 74 non-smoking mothers) and ways of delivery differed. In that study, neonatal distress (e.g. meconium within amniotic fluid, postnatal supplemention of oxygen), bacterial colonization or inflammation (e.g. positive vaginal smear, premature rupture of membranes) were not excluded, which may influence the investigated parameters. However, our study confirmed the previously found decrease in birth weight associated with maternal smoking .
According to literature [12, 20], mothers showed significantly lower TRECs and shorter telomeres than their neonates. Interestingly, newborns of SM demonstrated longer telomeres than newborns of NSM. Telomere lengths reflect the replicative history of each individual cell, which implicates a lower replicative rate in CD4 + CD45RA + T cells, in part also supported by a trend to higher TREC numbers in newborns of SM. A recent study showed significant cytogenetic damage of lymphocytes with changes of the mitotic index in neonates of SM . This is in agreement with the finding in our study of less replication of CD4 + CD45RA + T cells, as suggested by longer RTLs in this lymphocyte subpopulation. In neonates, CD4 + CD45RA + T cells may represent the naive T cell pool. In adults, not only naive T cells, but also CD45RA + CD28- effector T cells contribute to the CD4 + CD45RA + T cell pool . Usually in adults, these CD45RA + effector T cells are clonally exhausted and are negative for TRECS with short telomeres. However, CD45RA-expressing effector T cells can be neglected in newborns. Short telomere lengths have been associated with decreased lung function and increased risk of chronic obstructive pulmonary disease in adult smokers, with the association markedly attenuated after adjusting for age and other confounders . No effect of smoking on RTL was seen in SM compared to NSM in our groups. However, the mothers were younger than in that study  and had no history of an impaired lung function or chronic obstructive pulmonary disease. Additionally, as demonstrated by that study , several parameters confound telomere lengths in adults, thus, in contrast to the findings in newborns, making also our results from mothers difficult to interpret.
Cytokines were not altered in newborns of SM compared to NSM. Cytokine responses to allergens were tested in a study investigating newborns whose mothers smoked throughout pregnancy . In that study, higher but only partly significant responses for IL-13, IL-9, IL-5 and IL-6 were seen in newborns of smoking mothers, with no difference in IFN-γ responses. Similar to that study, the large variation in serum cytokines may not allow to draw clear effects of maternal smoking on newborn cytokine production in our cohort. SM and NSM showed higher IL-7 concentrations compared to their newborns, which may reflect the suggested role of IL-7 for proliferation of peripheral naive T cells to maintain the peripheral T cell pool in adults [25, 26].
Interestingly, SM had higher absolute lymphocyte numbers, which was also shown by others . In contrast to other studies [27, 28], proportions of CD3+ , CD4+ and CD8+ T cells, with an emphasis of CD4+ memory T cells , and B cells were not increased, as well as CD4 + CD25++ T cells were not decreased in SM in our study. Influences of dose, duration of smoke exposure and the ethnic background have been suggested to cause the heterogeneity in the findings , as well as redistribution of specific lymphocyte subpopulations from the periphery to other compartments, e.g. the bronchial lining associated with cigarette smoking [31, 32]. CD4 + CD25++ T cells do not necessarily consist of Tregs, but contribute also to the naive and activated T cell pool. Thus, for comparability with other studies , Foxp3 should be included in further studies to define truly regulatory T cells. Overall, no difference in the investigated lymphocyte subpopulations was seen between SM and NSM. These findings are similar to a study investigating acute effects of smoking and environmental tobacco smoke in five smokers and non-smokers , showing increased NK cells and decreased CD3+ and CD19+ cells which did not reach statistical significance. However, our results may be limited by the markers used to define the different lymphocyte subpopulations. Possibly, more differences between newborns of SM and NSM would have been revealed by investigating markers which are linked to more functional features of lymphocytes, such as expression of chemokine receptors, activation markers and intracellular cytokine production.
One strength of our study is the strict inclusion criteria, including only vaginally delivered term newborns with full maturity signs and no pre- or perinatal distress, to exclude changes of lymphocyte subpopulations caused by different gestational age, maturity or potential inflammatory mechanisms . However, one drawback of our investigation remains, that serum cotinine concentrations rather reflect acute exposure to cigarette smoke than its long term effects. Thus it could be that women, who did not smoke for a few days immediately before birth may have had inappropriately low serum cotinine concentrations whereas women passively or actively exposed during that time exhibited peak cotinine levels. Because of this and the known uncertainties of patient history taking, future studies should try to more adequately determine “true” cigarette smoke exposure, for example by measuring cotinine concentrations in the hairs from mothers and newborns .