The results of this study show that maternal undernutrition did not cause differences in the APP concentration in lambs at parturition, but that elevated levels of both Hp and SAA were present in all groups with the concentrations falling within 8–14 days. However maternal undernutrition in the HL group, reduced the response of SAA, but not Hp, to vaccination with Heptavac 7™. Apparent differences between groups for Hp concentrations (Fig ) were not significant due to large variances as shown by the error bars in Figures and . Therefore, whereas the restricted diet in the period leading up to parturition caused a reduction in the SAA response, undernutrition in early pregnancy as in the LH group in was not sufficient to affect this part of the innate immune system. This is the first report of the effect of maternal undernutrition on the ability of the acute phase response to contribute to the innate immune defence system but can be viewed as a further example of the potentially detrimental effect that maternal undernutrition has on offspring.
Recent research has demonstrated that maternal undernutrition has multiple and complex effects on the homeostatic and pathophysiological mechanisms in neonates and older offspring [16
]. A number of these mechanisms could contribute to the effect on the SAA response and also on the difference in SAA response between the first and second vaccinations. For example, maternal undernutrition leads to a reduction in liver size and a reduction in the expression of hepatocyte genes coding for proteins such as growth hormone receptor, prolactin receptor and suppressor of cytokine signalling-3 [17
]. As the primary site of synthesis of APP is the liver, following stimulation by pro-inflammatory cytokines, a reduction in liver volume or reduced expression of cytokine receptors could be expected to limit the production of the APP.
Effects of maternal undernutrition have also been observed on the hypothalamo-pituitary-adrenal axis [18
]. This has implications for the neuroendocrine-immune axis [21
] and this could also influence the responses of the APP to stimulants of the inflammatory and immune response such as the vaccine used here. There are reports of the influence of maternal undernutrition on the cortisol concentrations in offspring; for instance in rats with maternal undernutrition reduced production of cortisol in response to stress in offspring has been reported [18
]. Cortisol can have divergent effects on the acute phase protein response, acting synergistically with cytokines to activate the response or reducing the response by acting as an anti-inflammatory agent on macrophage and monocyte production of cytokines [22
]. Maternal undernutrition has also been shown in rats, to lead to a lower TNF response to endotoxin [19
] which would reduce production of APP, as seen here for SAA. Of interest is the observation, based on unpublished data, from the review by Lesage et al [18
] of a significant increase in C-reactive protein, the primary APP in humans, in adults born to undernourished mothers [18
]. We found no such significant differences in the pre-vaccination levels of SAA or Hp. However, the relatively small group size (n = 10) and high individual variation in the present study may have contributed to a lack of an observable effect of maternal undernutrition on the non-stimulated APP concentration.
In addition to the consideration of the effects of maternal undernutrition, the study provides further insight into the acute phase response in lambs. The fall in the concentration of Hp and SAA between the day of birth and the day of the primary vaccination (8–14 days after birth) has not been observed previously in this species. However raised levels of acute phase proteins in neonatal animals have been reported in calves and in piglets [23
] and may be due to the trauma of parturition or colonisation by environmental microbes.
Vaccination with Heptavac 7 caused an acute phase response in all animals. Stimulation of the APP response after vaccination has been reported previously [20
] when Hp was the only biochemical or immunological biomarker to show significant change after vaccination of calves with either antigen from multiple clostridial species or with Clostridium perfingens
types C and D toxoid. Here the study has been extended to quantification of the APP response to vaccination by measuring the reaction of both Hp and SAA and also by comparison of the responses in primary and secondary vaccinations. Monitoring the post-vaccination APP response may be a valuable investigatory tool, both for the examination of the role of the acute phase response in the innate immune system and for the investigation of early vaccine stimulation of the pro-inflammatory cytokines in relation to the efficacy of the vaccination process. Presumably, the immediate post vaccination acute phase response is stimulated by the pro-inflammatory cytokines which are known to be involved in the links between the innate and the acquired immune systems. Indeed research into vaccine adjuvants includes stimulation of the NFκ-B pathway which is also the target of acute phase inducing cytokines such as TNFα [26
The observation of a reduction in the dynamic response of SAA in the second vaccination, relative to that of the response to the primary vaccination, while the haptoglobin response was unchanged relative to the first vaccination, suggests that the control and mechanism of responses for these two proteins are different and could be explained if they are under the control of differing cytokine cocktails [29
]. The APP response is an integral part of the innate immune system and would be expected to have responded identically to both primary and secondary vaccinations, though little is known if this is true for stimulators given at 30 day intervals as here. For comparison further studies should include assessment of the response of lambs to a stimulator of sterile inflammation in the absence of vaccine antigens over a similar timeframe. Nevertheless differences were found in the APP response between the two inoculations which may relate to differences in the immunological status of the lambs at each vaccination. The lambs were immunologically naïve at the primary vaccination but not at the time of the secondary vaccination. The reduction in the dynamic acute phase response of SAA to the second vaccination could be explained by the fact that the lambs were partially immune on the latter occasion. It is known that, in humans, SAA is a type I APP, stimulated by IL-1-like cytokines, while Hp is a Type II APP being stimulated by IL-6-like cytokines [29
]. If similar mechanisms exist in sheep the primary vaccination could have stimulated both IL-1-like and IL-6-like cytokines while the secondary vaccination had a reduced effect on the IL-1-like cytokines and therefore a reduction the SAA response. Elucidation of the mechanism of differential stimulation of the response of Hp and SAA to primary and secondary vaccination will lead to greater understanding of the role of these APP, and their control by cytokines, in the host defences against infection.
It was not possible in this study to quantify the immune reaction but further investigation of the correlation between specific antibody or cell mediated immunity responses to the Heptavac vaccine and the immediate post-vaccination dynamic acute phase response of Hp or SAA is warranted. If the APP response was found to be correlated with subsequent immunity, monitoring the post vaccination acute phase reaction could have an important role in assessment of vaccine efficacy.