In this large birth cohort in Uganda, infant responses to BCG and tetanus immunisation were associated with pre-natal and early post-natal exposure to maternal M. perstans co-infection, maternal and infant HIV co-infection, and infant malaria. Maternal BCG scar showed associations with the infant response to BCG, and maternal immunisation with tetanus toxoid during pregnancy was associated with higher infant responses to their own tetanus immunisation.
As in any observational analysis, some findings may be explained by unmeasured confounders. However, most key factors identified were biological, rather than social or environmental, and adjustment for measured confounders produced little change in their effect estimates, suggesting that they are closely linked to causal mechanisms. Many statistical tests were conducted, so some apparently “significant” findings could have occurred by chance. Individual results are therefore treated with caution; rather than formally adjust for multiplicity, we focus on patterns and consistency of results, and on biological plausibility with reference to other findings.
Maternal M. perstans
microfilaraemia was associated with enhanced IL-10 responses to both cCFP and TT in the offspring. This filarial infection is highly prevalent in Africa and central South America, but usually asymptomatic [32,33]
. Adult worms inhabit serous cavities and microfilariae circulate in the blood, sometimes in thousands per millilitre, the lack of symptoms testifying to this helminth's potent immunoregulatory properties. Such helminth-induced regulation can influence host responses to unrelated antigens and IL-10 may be one key mediator of such effects 
; among other filariases, IL-10 responses to tetanus immunisation have been found to be elevated in adults with asymptomatic Onchocerca volvulus
. Our key observation is that the non-specific effect of helminths on this regulatory cytokine response can be transmitted from mother to infant. Notably, infant IFN-γ, IL-5 and IL-13 responses were not reduced, suggesting the possibility that protective immune responses may not be impaired, and it is possible that the overall impact of exposure to maternal helminth infection in utero is an enhancement of regulatory immune responses rather than suppression of the ability to mount protective responses to vaccines and pathogens. This might be broadly beneficial, protecting against excessive inflammatory responses, including allergy [36,37]
The lack of observed effects of maternal hookworm or S. mansoni
on type 1 and type 2 responses to mycobacterial antigens was surprising, given our own earlier findings 
, and those of Malhotra and colleagues .
However, in Malhotra's study all women had helminth infection: comparisons were made between infants sensitised and not sensitised to helminth antigens. Our study compared infants of mothers infected or not infected with each species, in a setting where most women had at least one helminth infection; moreover, for logistical reasons, a single stool sample was used for Kato Katz analysis giving limited sensitivity for diagnosis of intestinal helminths [39,40]
. Both these factors may have resulted in underestimation of effects of schistosomiasis and other intestinal helminths. However, taken together with the finding (reported elsewhere 
) that anthelminthics during pregnancy had little effect on infant responses to cCFP and TT in this study, these results suggest that maternal helminth infection may not be the major explanation for the poor efficacy of BCG immunisation in the tropics. Subsequent acquisition of helminths by the infant may be a different story 
Tetanus immunisation during pregnancy was associated with enhanced IFN-γ, IL-13 and (to some extent) IL-5 responses following tetanus immunisation of the offspring. These results accord with the earlier report of Gill and colleagues 
and show that priming of the infant response to TT can be influenced by immunisation of the mother. This antigen-specific effect may result from transfer of TT across the placenta within an immune complex, utilising the immunoglobulin receptor systems involved in transfer of maternal antibody to the fetus [42–44]
. Fetal exposure to antigen can result in tolerisation, but immune complexes are potent activators of the immune system, and this may explain why priming occurred in this case. The lower response to tetanus immunisation in HIV-exposed-uninfected infants may have resulted from reduced transfer of maternal antibody and antigen in this group [45,46]
By contrast, presence of a maternal BCG scar showed a negative association with infant type 2 cytokine response, and (to some extent) IFN-γ response to cCFP following BCG immunisation. This may have been a non-specific effect since maternal BCG scar was also associated with reductions in these cytokine responses to PHA (data not shown). The association was not explained by adjusting for potential confounding factors, and suggests an immunological interaction between mother and infant related to maternal mycobacterial exposure or infection. There is evidence for sensitisation to mycobacterial antigens in utero in mouse models and in humans [47,48]
, but tolerisation is also a possibility, and would accord with the lower response to mycobacterial antigen observed in Malawian, compared to British, infants following BCG immunisation 
. It may be important to investigate the role of maternal mycobacterial infection, and maternal immune responses to mycobacteria, in the infant response to BCG.
Current infant malaria and infant HIV infection were associated with broad reductions in IFN-γ, IL-5 and IL-13 responses. These findings were in keeping with the recognised immunosuppressive effects of these pathogens and thus, incidentally, demonstrate the ability of this immuno-epidemiological approach to detect important effects. They contrast with the IL-10-restricted effects of maternal M. perstans. For malaria, these effects were mainly short-term, observed only in the presence of current infection; however, decreased IL-5 cCFP and increased IL-10 TT responses were associated with both current and previous malaria. It is possible that the independent association between increased IL-10 TT responses and household socio-economic status might be mediated by repeated, unmeasured, exposures to infection.
Consistently lower responses were seen in girls. This shows that gender differences in immune response are present at an early age, and could be related to reported gender differences in the non-specific effects of immunisation on infant mortality 
This study examined factors influencing the cytokine responses induced by BCG and tetanus immunisation, not their efficacy. In the case of BCG, it is likely that IFN-γ is required, although not sufficient for, protective immunity 
, while excessive production of type 2 cytokines may be detrimental 
. Excess production of IL-10 may also be detrimental, if it is associated with suppression of protective responses, but evidence from the mouse model suggests that adequate production may be required to prevent a pathological, inflammatory response 
. Follow up of the cohort is in progress to determine how the observed responses are related to rates of M. tuberculosis
infection and disease. In the case of tetanus immunisation, the induction of neutralising antibody is key to protective immunity 
; the relationship between observed effects on cytokine responses and the production of antibody will be the subject of further investigation.From a public health perspective, our results demonstrate strong effects of current, or recent infant infections on the infant response to vaccine antigens, and reinforce the importance of control and treatment of malaria and HIV infection for the immunological health of mothers and their children; but suggest that maternal helminth infection may have little, if any, adverse effect on the outcome of infant immunisation. Immunisation during pregnancy may enhance the infant response to selected vaccines, and this, as well as the role of prior maternal BCG immunisation and mycobacterial infection in determining the infant response to BCG immunisation, needs to be explored in further research.