Results
The F line had a lower response to Con A than the RBC2 in peripheral blood mononuclear cells and whole blood (Li et al., 1999a). There were no significant line differences in response to PHA-M for both assays. For the purified peripheral blood mononuclear cell assay, the F line had a lower response than the RBC2 line to Con A expressed as either cpm or a stimulation index (ratio of cpm for stimulated cells to the cpm for unstimulated cells). For the whole blood assay, the F line had generally lower stimulation index values in response to Con A than the RBC2 line, with differences being significant at 0 and 2 days postinjection (P < 0.01) and at 14 days postinjection (P < 0.05).
CD4 and CD8 T Cell Populations
The CT4 mAb cross-reacted with all turkey CD4 molecules regardless of the line (Table 1; Li et al., 1999b). However, the CT8 mAb did not detect the CD8a molecule in some individuals and the frequency of identification showed distinct line differences. After staining with the CT8 mAb, peripheral blood lymphocytes from all turkeys from the Commercial B line were CD8a positive. The CT8 mAb did not detect the CD8a+ T cells in some turkeys from Lines RBC2 (4 /56), Commercial A (4/18), and Commercial C (1/10). About half of the turkeys in Line F had no CD8 population (24/47) as detected with the CT8 mAb.
Because the CT8 mAb appeared to recognize a polymorphic determinant on the turkey CD8a molecule, a panel of mouse anti-chicken CD8a mAb was subsequently tested on five turkeys (Li et al., 1999b). Among them, two turkeys from Line F and one from Line RBC2 had no CD8 subpopulation (CT8-) as detected with CT8 mAb previously, whereas one turkey from each line with positive result (CT8+) was also included. These five turkeys were all positive as detected with CT4 mAb. Three mAb (3-298, 3-292, and 11-39), which recognize chicken CD8a, had good cross-reactivity with turkey peripheral blood lymphocytes, and a clear-cut region was found between positive and negative cell populations with single color staining (Table 2), whereas all other chicken CD8a specific mAb did not cross-react. The EP42 mAb that recognizes the chicken CD8b molecule had weak cross-reactivity with the turkey peripheral blood lymphocytes (PBL). Among a panel of the mAb that recognize chicken CD4 molecule, most (the 2-35, 2-6, and 7-125 mAb) also had very good cross-reactivity with turkey PBL cells and had distinct populations. Only one CD4 mAb (10-3) did not cross-react. Polyclonal antibodies were also cross-reactive with turkey peripheral blood lymphocytes (data not shown), but did not have populations as clear as those obtained with the mAb.
Thereafter, the 3-298 mAb was tested on more turkeys in three different lines. The 3-298 mAb seemed to cross-react with all turkey PBL in Lines RBC2 , F, and Commercial C (Table 1). Immunoprecipitation and Western blotting showed that the 3-298 mAb precipitated a 33- to 35 kDa polypeptide.
Based on the cross-reaction of the CT8 mAb with peripheral blood lymphocytes, turkeys in each line were divided into two groups: CT8 + and CT8 - (Li et al., 2000). Within each line and sex subgroup for the F and A lines, there was no significant difference in body weights at 8, 16, and 20 weeks of age and shank parameters (length, width, and depth) at 16 weeks of age between the CT8 + and CT8 - groups. However, significant differences were observed between the CT8 + and CT8 - groups for body weights at 8, 16, and 20 weeks of age, and shank width at 16 weeks of age in female turkeys from the RBC2 line.
The proportion of CD4 and CD8 defined cell subsets in turkey peripheral blood lymphocytes from the three lines is given in Table 3. At 16 weeks of age, turkeys in the F line had a significantly higher percentage of CD4+CD8- subset and a higher ratio of CD4+CD8- ( helper T cells) to CD4-CD8+ (cytotoxic T cells) (CD4:CD8 ratio) than the RBC2 line (P < 0.05). No significant line difference was observed in the other cell populations. At 24 weeks of age, turkeys in the F lines still had a higher percentage of CD4+CD8- subpopulation than the RBC2 line. Compared with the F line, the Commercial Sire Line B line had a significant higher percentage of the CD4+CD8+ subset, but lower percentages of CD4-CD8- and CD4+CD8- cell subsets as well as a lower CD4:CD8 ratio. No significant differences except in the percentage of CD4+CD8+ subsets were observed between the RBC2 and B lines at 24 weeks of age.
Serum Immunoglobulin Concentrations
There were line differences in serum IgM concentrations (Table 4; Li et al., 2000a), but IgG concentrations did not differ between lines. Serum IgM concentrations were significantly higher in Line F turkeys compared with Line RBC2 (P < 0.01 for Trial 1; P < 0.05 for Trial 2). In addition, the F-line poults had higher mortality (85.4 vs 61.1% on average) and died earlier (6.32 vs 10.49 days on average) than the RBC2-line birds after challenge with P. multocida (P < 0.01). The Pearson correlation coefficients indicated that there was no association between days to death and either IgG or IgM concentration (coefficients ranged from -0.227 to 0.030, P > 0.05). There were significant (P < 0.001) correlation coefficients between serum IgG and IgM levels in Trial 1 (0.498) and Trial 2 (0.547).
Antibody Responses to Various Antigens
Compared with the RBC2 line, the F line had generally higher total anti-sheep red blood cell (SRBC) titers; the differences were significant at 14 days post-primary immunization (females) and 10 days post-secondary immunization (males) and at 4, 7, and 10 days post-secondary immunization (females) (Li et al., 2000b). For IgG titers, the F line had higher titers than did the RBC2 line at 4 and 10 days post-secondary immunization. In general, there was no line difference in response to B. abortus antigen (Li et al., 2000b) and Newcastle disease virus and P. multocida vaccines (unpublished data).
Phagocytic Activity
Phagocytic activity was evaluated by the carbon clearance assay. The F line had lower phagocytic activity than the RBC2 line. More carbon particles remained in the blood of the F-line turkeys than in the RBC2 turkeys at 3 and 6 minutes post-injection with India ink indicating that the F-line individuals cleared carbon particles from the circulation at a slower rate than the RBC2-line birds. There was no sex difference in carbon clearance.
Changes in Lymphoid Organ Weights
The weights of the spleen and bursa of Fabricius were measured at 9 weeks of age and expressed relative to total body weight. The F line had greater body weight, relative weight of the spleen, and ratio of spleen to bursa of Fabricius weight but had lower relative weight of the bursa of Fabricius.