Can immunocrit be used as a monitoring tool for swine vaccination and infection studies?

Abstract

Background: The immunocrit is a cost-effective and straightforward technique traditionally used to assess passive immunity transfer to newborn piglets. However, it has not been previously used for monitoring the effect of vaccination and/or infections. Therefore, this study aimed to evaluate the usefulness of the immunocrit technique as an immunological monitoring tool in a vaccination and challenge scenario, using porcine circovirus 2 (PCV-2) as pathogen model. The immunocrit ratio was monitored in PCV-2 vaccinated (V) and non-vaccinated (NV) 3-week-old piglets (study day 0, SD0) that were subsequently challenged with this virus at SD21 and followed up to SD42. Additional techniques (PCV-2 IgG ELISA, optical refractometry, and proteinogram) were performed to further characterize the results of the immunocrit analysis.

Results: Immunocrit, γ-globulin concentration and PCV-2 S/P values followed similar dynamics: descending after PCV-2 vaccination but ascending after an experimental PCV-2 inoculation. However, statistically significant differences between V and NV animals were only found with the PCV-2 ELISA. In this case, V animals had significantly higher (p < 0.05) S/P values (S/P ratio = 0.74) than NV (S/P ratio = 0.39) pigs only after challenge at SD42. On the other hand, serum total protein obtained by refractometer (STPr) were maintained from SD0 to SD21 and increased in both groups from SD21 to SD42. Correlations between techniques were low to moderate, being the most robust ones found between immunocrit and optical refractometry (ρ = 0.41) and immunocrit with γ-globulins (ρ = 0.39). In a subset of sera, the proteinogram technique was applied to the whole serum and the supernatant of the immunocrit, with the objective to characterize indirectly the immunocrit fraction. The latter one included all protein types detectable through the proteinogram, with percentages varying between 64.3% (γ-globulins) and 82% (β-globulins).

Conclusion: The immunocrit technique represented a fraction of the total serum proteins, with low to moderate correlation with all the complementary techniques measured in this study. Its determination at different time points did not allow monitoring the effect of vaccination and/or infection using PCV-2 as a pathogen model.

Keywords: Immunocrit; Infection; Monitoring; Porcine circovirus 2; Total serum protein; Vaccination.

Fig. 1

Scheme of the study design. NV: not vaccinated (received PBS intramuscularly); V: vaccinated; SD: study day; STP: serum total protein; woa: weeks of age

Fig. 2

Results of the performed techniques at different time-points for V and NV animals. A immunocrit, B PCV-2 IgG ELISA S/P ratio, and C STPr (obtained by optical refractometry). For each technique, different uppercase letters indicate statistically significant differences between V and NV for each particular study day, and the lowercase ones indicate statistically significant differences within a group between study days (p < 0.05). V: vaccinated; NV: not vaccinated; SD: study day; STP: serum total protein

Fig. 3

Results of different parameters obtained with the proteinogram technique at different time-points for V and NV animals. A STPp, B albumin, C α-1 globulin, D α-2 globulin, E β globulin, and F γ globulin concentration (g/dl). For each parameter, different uppercase superscript letters indicate statistically significant differences between V (bold) and NV (dotted) for each particular study day, and the lowercase ones indicate statistically significant differences within a group between study days (p < 0.05). V: vaccinated; NV: not vaccinated; SD: study day; STPp: serum total protein obtained by proteinogram technique

Fig. 4

Spearman’s correlation matrix between parameters: immunocrit ratio, STPr, PCV-2 IgG ELISA S/P ratio, γ-globulin concentration and STPp. STPp: serum total protein obtained by the proteinogram technique; STPr: serum total protein obtained by optical refractometry. The asterisk symbol (*) indicates that the correlation among the analyzed parameters is statistically significant (p < 0.05)

Fig. 5

Comparison of protein concentrations (measured by the proteinogram technique) of complete serum (prior precipitation) and the supernatant after immunocrit technique (following precipitation with 40% (NH₄)₂SO₄). Protein concentrations of the precipitate were inferred by substracting the protein concentration of the supernatant (17.2–35.7%) from the protein concentration of the complete serum (100%). Concentration values are expressed g/dl