Infectious diseases remain among the major causes of illness and death worldwide. There is mounting evidence that developmental exposure to common chemicals from our daily environment represents an overlooked contributor to differences in host responsive capacity to infections 
. Using the well characterized immune response that develops following influenza A virus infection, we created an assessment matrix that not only investigated the possible link between maternal exposure to BPA and altered host defenses against respiratory infections, but examined the effects of developmental exposure to BPA on several measures of innate and adaptive immune function known to play key roles in combatting infection. In many ways, our findings should be perceived as good news. Although concerning effects of low level exposure to BPA on reproductive cancers have been reported 
, our findings indicate that the function of key cellular components of anti-viral adaptive immunity are largely unaffected by developmental exposure to BPA. Correlating with no change in the host’s ability to mount adaptive immunity against influenza virus infection, BPA exposure during development did not adversely affect host resistance to primary or recall infection. Based on these observation it appears that, in contrast to several other pollutants, such as dioxins and polychlorinated biphenyls, tobacco smoke, and lead 
, developmental exposure to BPA does not have significant influence on the magnitude or functional capacity of the antigen-specific adaptive component of the immune system.
Others have reported that early life BPA exposure correlated with an increase in lymphocyte functions, such as elevated Th2 cytokine production following a bacterial infection, or enhanced T cell and antibody responses following immunization with foreign proteins 
. One possible explanation for these differing observations is that BPA acts on cellular mechanisms important for driving Th2-polarized, but not Th1-polarized, immune responses. That is, the effects of BPA on the immune system could mimic the effects of estrogens, by skewing the Th1/Th2 balance in favor of Th2 responses 
. However, the idea that estrogens polarize toward Th2-biased responses is likely an oversimplification of the physiological and pharmacological effects of endogenous and exogenous estrogens on immune function 
, and therefore is probably not an adequate explanation. Other explanations for differences between our findings and prior reports include the dose administered, as well as the timing and route of exposure. Some of the studies in which altered immune function was reported used doses of BPA that were 10–100 fold higher. This may be a critical difference, as mice treated in our study had circulating BPA levels on par with levels reported in the general population. Moreover, in some of the other studies lower doses of BPA were used, and reported to have no effect on ex vivo
immune cell functions or disease progression in vivo
. Similarly, direct exposure of mature animals to BPA, often in the mg/kg/day dose range, affected proliferation and cytokine production by ex vivo
re-stimulated lymphocytes, but in vivo
exposure in the µg/kg/day range did not alter these metrics or affect disease progression 
. Given that there are reports of BPA exhibiting non-monotonic toxicity, is important to note that we also examined the consequences of maternal exposure to lower doses of BPA [0.5 and 5 µg of BPA/kg body weight/day] administered in the same dosing paradigm as the studies reported herein. We did not observe any differences in the primary immune response to influenza A virus infection in mice developmentally exposed to these doses of BPA (data not shown). Thus, the dose of BPA administered, particularly doses above 50 µg/kg/day, may modulate immune cell behavior, but there is no consensus on the relationship between doses to rodents in the mg/kg/day range and current human exposure.
Regulatory T cells can suppress the CD8 T cell response during virus infections 
. It has been also reported that developmental exposure to BPA alters the magnitude of the regulatory T cell population 
. Using a transgenic mouse strain, one study showed that developmental exposure to BPA induced oral tolerance, and suggested that BPA might have effects on T cell selection and activation processes that involve clonal deletion and regulatory T cells 
. However, in the current study, we did not observe an effect of developmental exposure to BPA on the number of CD4 T cells or CD4+
regulatory T cells in the MLN or lung. This finding is consistent with our observation that the overall CD8 T cell and antibody responses to infection did not differ between treatment groups. It is important to note that although the specific CD8 T cell population detected 9 days after infection may appear to be of greater magnitude in BPA exposed animals, no statistically significant differences in the mean percentage or number were observed in this or in independent repeats of this entire experiment. Thus, we conclude that the subtle changes observed in BPA exposed animals are unlikely to indicate a modification in the functional aspects of adaptive immunity that mediates virus clearance. In addition to the primary T cell response, our data also showed that developmental exposure to BPA did not compromise the ability to generate or execute an effective T cell based memory immune response. To our knowledge, this is the first demonstration of the consequences of developmental exposure to BPA on T cell based memory to an infection. Assessment of memory immune responses has implications beyond virus infections. For example, vaccines are intended to generate host protective immunological memory. An increased susceptibility to infection associated with exposure to environmental agents could result from reduced vaccine efficacy. Our data suggest that developmental exposure to BPA most likely would not interfere with efficacy of any vaccine intended for generating long-term T cell memory. Although we did not test antibody-based immune memory directly, our data show that development exposure to BPA had no effect on the ability of B cells to produce antibodies or undergo isotype class switching; from which we infer that a detrimental effect of early life exposure to BPA on antibody-based memory is also unlikely.
Although maternal BPA exposure impaired neither the magnitude nor the effectiveness of adaptive immunity, we did observe a reduction in pulmonary inflammation and cytokine expression levels in the lungs seven days following infection. This may also be associated with the variation seen in the lung virus titer of the mice. These observations suggest that developmental exposure to BPA may transiently diminish aspects of the inflammatory response to infection. Seven days post influenza virus infection in mice, two major lung infiltrating leukocyte populations are neutrophils and macrophages, and production of TNF-α and iNOS are hallmark functions of these cells 
. Diminished expression of these genes in the infected lung suggests that developmental exposure to BPA might have some effects on inflammatory mediators, which can be produced by leukocytes or the structural cells of the lung. Several in vitro
studies have shown that exposure to BPA diminishes functions of neutrophils and macrophages, including the production of TNF-α and nitric oxide 
. An influence of maternal BPA exposure on cytokine production has also been reported in several animal models 
. Given that the level of inflammatory cytokines is associated with the physical malaise experienced during influenza virus infection 
, it is possible that changes in cytokine production influence disease severity. However, using body weight as a measure of morbidity, we observed no evidence for this idea using two different strains of influenza virus. Interestingly, during the early days after infection, which is dominated by innate immune mediators, we observed slightly higher lung virus titers in some of the animals that were developmentally exposed to BPA. Although not statistically significant, this was observed in more than one study. Such a difference could be a consequence of reduced inflammation in the lung at that time, since early innate immune responses play a critical role in containing the virus during the early stages following infection 
. At present we do not know whether the influence of BPA on lung inflammation is a result of direct effects of BPA on hematopoietic stem or progenitor cells that are exposed during development, or the result of an indirect effect of BPA, via endocrine or other systems that interact with hematopoietic or lung cells, and which could have been altered by developmental exposure to BPA. Moreover, whether and how early life exposure modulates diseases in which tissue inflammation plays a role in initiation of pathology remains to be determined. Our observations in the influenza-infected lung suggest that early life exposure may lead to attenuated rather than enhanced tissue inflammation.
To our knowledge, the current study is the first comprehensive investigation on the effects of developmental exposure to BPA on host resistance that integrates different aspects of immunity and quantitatively evaluates those parameters in a disease-specific manner. We demonstrate here that developmental exposure to low doses of BPA had only mild effects on the immune response to influenza A virus infection, and did not compromise the ability of the infected host to successfully clear the virus. Evidence exists that exposure to certain environmental agents can selectively affect some components of the immune system, and may even show pleiotropic effects on certain aspects of the immune response 
. Since the immune system is composed of multiple cell and tissue types, each having specific but overlapping contributions to overall host resistance, it is not surprising to observe subtle immunomodulation in one component of the antiviral response without an overall effect on virus clearance and disease outcome. Given that high doses of BPA have been shown to act as xenoestrogens in-vitro
, and considering the intricate relationship between the immune and endocrine systems, it is plausible that some BPA-mediated endocrine disruptive mechanism might be responsible for the transient depression in innate immunity that we observed following influenza virus infection. Alternatively, BPA could also have direct effects on cells that mediate innate immunity, such as down regulation of NF-κB pathways, as suggested by other in vitro
. Although the transient reduction in innate immunity did not negatively affect influenza virus clearance, in a different scenario wherein the inflammatory responses is critical for early pathogen clearance (e.g., certain bacterial infections), a reduction in innate immune functions from BPA exposure might prove detrimental. On the other hand, during innate immune-mediated diseases, such as chronic inflammation, BPA-mediated reductions in innate immune functions might have beneficial effects. While all these ideas are experimentally testable, the totality of our findings suggest that developmental exposure to low oral doses of BPA did not compromise the key aspects of anti-viral immunity to influenza virus infection.