Thermal influences on antibody production and metabolism in chicken lines divergently selected for immune responsiveness

IntroductionThe international character of the poultry industry requires that poultry bred under temperate conditions should be able to perform under tropical conditions as well. However, abrupt changes in environmental temperature or continuous high temperatures may affect production traits, immune responsiveness, and thus disease resistance.Improved disease resistance can be obtained by breeding for high immune responsiveness. However, if the effects of a stressor, e.g. high temperature, is be different in the successive selection lines, the selective advantage can be lost.Two mechanisms might cause such a genotype x environment interaction. First, the stress susceptibility may be increased after the selection for high responsiveness. Second, the body weight, and thus possibly metabolic rate, may be different between the selection lines. This could imply different thermal requirements of the birds, and thus different reactions to changed environmental temperatures.In the present studies, the effects of acute thermal stress and continuous high temperature on antibody production to sheep red blood cells (SRBC) were investigated in young chickens, which had been selected for either high (H) or low (L) antibody titers to SRBC for several generations. Also, the influence of infused corticosterone, the "stress "hormone that is held responsible for immunosuppression, was studied. The general aim of the studies was to investigate the occurrence of genotype x environment interactions and the causes for such interactions.Chapter ITo study the effects of acute thermal stress on antibody production, three experiments were carried out. The chickens were subjected to an acute heat stress treatment (4 periods of½h at 42°C, with ½ h intervals) 24 h before an immunization with SRBC. Experiments reported in literature described considerable immunosuppressive action of such treatments. This treatment could give therefore a good opportunity to study the possible differences in response between the selection lines.Immunizations were given intramuscularly (i.m.) and intravenously (i.v.) (with different doses). Only in one experiment was a relatively small, but significant, immunosuppressive effect found in the H line, but not in the L line. The immune response in the H line was always considerably higher than in the L line.Chapter IITo quantify the impact of the heat stress treatment on the birds, changes in a number of physiological parameters were studied in the birds, which were subjected to the treatment described in chapter I.During the heat stress treatment cloacal temperature and plasma corticosterone were increased. Changes in numbers of circulating leukocytes, and plasma albumin and fibrinogen concentrations were not changed during the heat stress, but changed during the following days in which the immune response was developing. Differences in these changes between i.m. and i.v. immunized birds were found, but H and L line were not different.Chapter IIIIn chapter III an experiment is described, in which the chickens were placed in climate chambers. In this experiment, an acute thermal stress was given, comparable to that described in chapters I and II, but the effects of prolonged heat on the birds were also studied. After acclimation to high temperature, the influence of acute stress diminishes, and endocrine and metabolic changes can affect antibody production. The environmental temperature was constant either 25° or 35°C or it fluctuated daily between 15-25°C or 25-35°C. At the time the immunization with SRBC was given, some of the chickens were exchanged between the different temperatures (acute stress).At the high temperature, the chickens were hyperthermic, showed depressed growth, and post mortem lower weights of lymphoid organs were found equally in acclimated and non-acclimated chickens. Birds moved from 25° to the 35°C environment (CH) had a lower immune response than those that remained at 25° (CC). The birds which were already in the 35° environment before immunization (HH) mounted higher immune responses. The responses in birds which were kept in the chambers with fluctuating temperatures, showed less pronounced differences, suggesting a smaller impact of these temperatures, or a better acclimation of the birds to the temperature. These effects were the same in H and L line chickens.Chapter IVA number of experiments are described in chapter IV, in which the ontogeny of lymphoid organs (important for the immune system), and the number of plaque-forming-cells (the precursors of the antibodysecreting cells) in the spleen were studied. Aim of this study was to find morphological differences between the H and L line that might determine the difference in immune responsiveness. It was found that the spleen was already heavier in the H line than in the L line, before immunization. Also, higher numbers of plaqueforming-cells were found in the spleen of birds from the H line than in those of L line birds after immunization. These differences contribute to the differences in antibody production after immunization.Chapter VBecause the effects of acute heat stress on antibody production, described in chapters I and III, were smaller than expected, a study with direct application of corticosterone was done. Corticosterone is the hormone presumed to cause immunosuppression after stress. Direct administration of corticosterone might reduce the variability in the response measured after thermal stressors, and thus provide a more precise comparison of stress effects between the lines. In chapter V this experiment is described in which the birds were infused with a corticosterone solution (CS).Major effects of the CS treatment on plasma corticosterone, growth, leukocytes and lymphoid organs were found. The antibody titers to SRBC were unaffected, in the H and L line.In chapters VI, VII and VIII the effects of the treatments already described in chapter III, (CC, HH and CH) and V (CS infusion) were studied more extensively. Changes in metabolic rate, growth, energy and protein turnover, caused by the different temperature treatments might have comparable effects on antibody production as obtained by selection. These treatments were studied in relation to changes in endocrine factors (corticosterone, growth hormone, somatomedine, thyroid hormonesT 3andT 4 )and energy metabolism. Therefore these studies were performed in the climate-respiration chambers.Chapter VIBecause an average weight difference exists between the H and L line (L line is usually about 7% heavier) the influence of differences in metabolic rate between the lines on antibody production was studied.Heat production was recorded at different temperatures in both lines. Heat production was somewhat higher in the L line than in the H line in one of two experiments, but no lower critical temperature could be estimated in either line. Thermo-neutrality was maintained by adjusting feed intake.During the immune response only minor differences in energy metabolism were detected between the two lines. These could not unanimously be related to the height of the immune response.Chapter VIIChickens were subjected to a CC, CH or HH treatment, as described in chapter III. The effects of high temperature on metabolism were very evident. Decreased feed intake, growth rate, heat production and fat deposition were found. In the CH treatment also a decreased protein retention was found during the balance period immediately after the temperature change. Effects on cloacal temperature and lymphoid organs were as reported before (Chapter III). The effects of the temperature treatments on antibody production were not as impressive as found in the experiment in chapter III. Plasma corticosterone and thyroid hormones were not affected, somatomedine was increased and growth hormone decreased in hot environments. Differences in metabolic traits, caused by the different environmental temperatures were not related to differences in antibody titers, and differences in antibody titers between the H and L line were not reflected in differences in metabolic rate.Chapter VIIIFinally, the CC and HH treatments were compared to CS infusion. Some effects of the infused corticosterone and HH treatment were similar. Decreased feed intake, growth and heat production were found in both treatments. Also lymphoid organs were negatively influenced in either treatment. But no increased plasma corticosterone levels were evident in HH treatment.T 4concentration was decreased in CS infused birds;T 3was lower in CS infused and was the lowest in HH treated birds. The energy balance was higher in CS infused birds immediately after immunization. Fat deposition was higher in CS infused birds. A small stimulatory effect of the HH treatment on antibody production was found in the L line birds only. Similar to chapter VII, no clear relations between endocrine parameters, energy metabolism and antibody production were found. It was questioned whether the CS infusion is valuable as an experimental model for continuous (heat) stress, because of the marked differences in a number of the measured characteristics.DiscussionIn the discussion it is argued that the differences between H and L line in antibody production are very steady.IFrom the experiments, it is evident that severe heat stress, prolonged heat and corticosteroids do affect the birds: growth, body temperature, corticosterone and other hormones, lymphoid development are changed, but equally in both lines. The selection did not result in a "high stress susceptibility line".Moreover, both lines are apparently rather stress resistant, with regard to antibody production. Acute heat stress, prolonged severe heat and evencorticosterone infusion could only marginally affect antibody titers. If the antibody production was affected by the experimental treatment, the changes were usually very similar in the H and L line. The absence of genotype x environment (heat stress) effects was therefore apparent.The influence of metabolic rate, as measured here, on antibody production was rather small. Some changes in fat deposition during the balance period immediately after the immunization were found, which could indicate a shift in energy distribution between fat and protein, during the immune response. But no direct relation between metabolic rate, influenced by selection or environmental temperature with endocrine regulation and antibody production was found. It was also concluded therefore that the difference in bodyweight between the selection lines is rather based on linked genes than physiologic meaningful relations between the immune system and energy metabolism.