Age at weaning, immunocompetence and ectoparasite performance in a precocial desert rodent.

We studied the effects of early weaning on immunocompetence and parasite resistance in a precocial rodent Acomys cahirinus. We hypothesized that if parasite resistance is energetically expensive and nutritional and immunological support from mothers are necessary for the long-term health of offspring, then early weaned animals would be immunologically weaker and less able to defend themselves against parasites than later weaned animals. We weaned pups at 14, 21 or 28 days after birth and assessed their immunocompetence and resistance against fleas Parapulex chephrenis when they attained adulthood. Immunocompetence was assessed using leukocyte concentration (LC) and a phytohaemagglutinin injection assay (PHA test). To estimate resistance against fleas, we measured performance of fleas via the number of produced eggs and duration of development and resistance to starvation of the flea offspring. We found a significant positive effect of weaning age on the PHA response but not on LC. The effect of age at weaning on flea egg production was manifested in male but not female hosts, with egg production being higher if a host was weaned at 14 than at 28 days. Weaning age of the host did not affect either duration of development or resistance to starvation of fleas produced by mothers fed on these hosts. We conclude that even in relatively precocial mammals, weaning age is an important indicator of future immunological responses and the ability of an animal to resist parasite infestations. Hosts weaned at an earlier age make easier, less-resistant targets for parasite infestations than hosts weaned later in life.

CD4+ T cells dominate the leukocyte infiltration response initiated by intra-dermal injection of phytohemagglutinin into growing feathers in chickens.

Phytohemagglutinin (PHA) is commonly used to evaluate cell-mediated immunocompetence. In chickens, PHA is typically injected intra-dermally (i.d.) into the skin (e.g., wing web, wattle, or footpad), and the tissue swelling response is monitored, whereby the extent of tissue swelling positively relates to the individual's cell-mediated immune system capabilities. Although i.d. injected PHA was shown to stimulate mononuclear cell and basophil infiltration to the site of injection, reports on temporal, qualitative, and quantitative aspects of the local cutaneous PHA response are limited. The objective of this study was to use the growing feather (GF) as a cutaneous test site to assess and monitor the type and relative amounts of leukocytes present in the pulp of PHA-injected GF. For this study, male, non-vaccinated Light-brown Leghorn chickens reared at the Arkansas Experiment Station Poultry Health Laboratory were used. At 9 wk of age, the dermis of 20 18-day-old regenerating GF was injected with 10 µL of either PBS diluent or 300 µg/mL PHA-P (5 chickens per treatment). GF were collected from each chicken before (zero) and at 0.25, 1, 2, 3, 4, 5, and 7 d post injection. At each time point, one GF was collected for immunofluorescent staining of pulp cell suspensions and leukocyte population analysis by flow cytometry, and another GF for histological analysis. Histological analysis confirmed participation of granulocytes and mononuclear cells, primarily lymphocytes, in the cutaneous PHA response. As revealed by flow cytometric cell population analysis, T cells, especially CD4+ T cells, constituted the major portion of the mononuclear cell infiltrate. Levels of CD4+ T cells were greatly elevated in PHA-injected GF within 6 h and remained elevated throughout the 7-day examination period. γδ T cells, CD8+ T cells, and B cells also infiltrated in response to PHA although at lower levels and with different time-course patterns from CD4+ T cells. The dominant presence of CD4+ T cells at the site of PHA injection further affirms this polyclonal response as an indicator of cell-mediated immunity.