Published online by Cambridge University Press: 13 March 2007
Genetic selection is well established as a means of improving productivity in pigs, but the effects of continued selection for increased performance on immunity are not well understood, nor are genetic relationships between performance and immunity. This study compared differences in the levels of a range of immune traits between lines of Large White pigs divergently selected for a number of productivity traits. Selection lines compared were high v. low lean growth under restricted feeding (31 high line v. 10 control v. 38 low line pigs), high v. low lean growth under ad libitum feeding (18 high line v. 10 control v. 19 low line pigs), and high v. low food intake (24 high line v. 26 low line pigs). Immune traits measured were total white blood cell numbers (WBC), and the numbers of leukocyte subsets: neutrophils, monocytes, eosinophils, lymphocytes, CD4+ cells, CD8α+ cells, B cells, γδ T cells and CD11R1+ Natural killer (NK) cells. CD4+, γδ T cells and CD11R1+ cells were subdivided into subpopulations that were positive or negative for the CD8α marker, and conventional CD8αhigh+ cytotoxic T cells were also determined. Pigs were tested under ad libitum feeding conditions from 14 to 24 weeks, and immune traits were assessed at ages 18 and 24 weeks. Line differences were estimated using residual maximum likelihood techniques. Consistent differences in immune trait levels were evident between pigs previously selected for high and low lean growth under restricted feeding: at age 24 weeks, high line pigs had higher basal levels of WBC (39·6 v. 27·8×106 cells per ml, s.e.d. 2·09, for high v. low line pigs) mainly explained by higher levels of lymphocytes (25·5 v. 17·3×106 cells per ml, s.e.d. 1·54, for high v. low line pigs) with increased numbers of CD8α+ cells (8·19 v. 5·15×106 cells per ml, s.e.d. 0·14) and CD11R1+ cells (5·23 v. 2·46×106 cells per ml, s.e.d. 0·43), predominantly the CD11R1+ CD8α? subpopulation ((3·20 v. 1·64×106 cells per ml, s.e.d. 0·11). High line pigs also had increased numbers of monocytes (2·64 v. 1·83×106 cells per ml, s.e.d. 0·35). Similar results were obtained at age 18 weeks. There were no consistent differences between divergent lines in pigs selected for lean growth under ad libitum feeding or food intake. This is the first report to demonstrate that selection for some aspects of performance can influence WBC and leukocyte subset numbers in pigs.