Immune Response to Newcastle Disease Virus Vaccination in a Wild Passerine

Because parasites and pathogens can have a critical effect on individual fitness, protection against disease is of major importance. However, pathogen avoidance and investment in immune function often interact with other vital parameters, and even nonpathogenic immune challenges can induce relevant physiologic changes in the host (Norris and Evans, 2000). Most of our knowledge about the function and dynamics of immune responses comes from laboratory studies in controlled environments or from studies on inbred individuals of a small number of model species. In contrast, natural populations exhibit wide genetic and environmental diversity, and epizootic diseases often involve vectors and hosts of a variety of species that interact in a complex way (Pedersen and Babayan, 2011).

Adaptive immunity is the most efficient line of defense in vertebrates, as it targets specific antigens, relying on both humoral and cell-mediated immunity to ensure a faster response to subsequent challenges. The humoral response is mediated by lymphocytes T and B that recognize specific antigens, and secrete antibodies to target them for destruction. Likewise, cell-mediated immunity relies on a variety of immune cells that are responsible for antigen recognition and memory, production of antibodies, and the elimination of the specific antigen, among other functions. However, little is known about variability in the early dynamics of those responses and their long-term persistence, which limits their suitability for evolutionary and ecologic studies in the wild.

Immunologic research on wild avian systems has been mostly restricted to nonpasserine species, and the use of antigens such as sheep red blood cells (SRBC) or keyhole limpet hemocyanin (KLH), to which birds have never been naturally exposed. However, the study of adaptive responses to specific wild pathogens can yield relevant information on epidemiologic and evolutionary issues concerning host–pathogen dynamics (Demas et al., 2011).

Newcastle disease virus (NDV) is a globally distributed avian paramyxovirus that causes highly contagious disease and represents a severe problem for the poultry industry (Alexander, 2009). Few studies have been performed on the specific immune response to NDV, especially in wild populations and nonmodel species (Seal et al., 2000). Vaccination is a common technique widely applied to prevent and alleviate disease and as an immunologic tool to study eco-epidemiology in wild animal populations (e.g., Staszewski and Boulinier, 2004). The development of new in vivo immunologic methods can improve our understanding of host–pathogen dynamics in the wild, potential pathogen reservoirs, and epizootics that may be crucial for endangered species' survival, and the study of passive immunity (e.g., Garnier et al., 2011).

We assessed the immune response of wild House Sparrows (Passer domesticus) to immunization with NDV inactivated vaccine. In particular, we aimed to determine the dosage eliciting a significant immune response and the temporal pattern of cell-mediated and humoral responses in adult House Sparrows. House Sparrows are commonly exposed to a variety of pathogens in the wild and are sometimes considered a source of epizootic threats because of the species' close association with humans and livestock (Leighton and Heckert, 2007).

We captured 81 adult House Sparrows in December 2009 in the vicinity of Cañada de los Pájaros bird reserve in Puebla del Río, Sevilla, Spain (37°14′N, 6°07′W). Birds were housed in outdoor cages (3×3×2 m) with ad libitum food and water. On day 2, blood samples (0.2 mL) were taken and individuals were randomly assigned to five treatments: 1–3) inoculated subcutaneously with different doses of a commercial inactivated Newcastle disease virus vaccine HIPRAVIAR® BPL2 10^9.5 EID₅₀/0.5 mL (0.2 mL, 0.1 mL, or 0.05 mL), 4) inoculated with the same oil adjuvant (0.1 mL) as the commercial vaccine, or 5) inoculated with sterile phosphate buffered saline (0.1 mL). Sampling was repeated at 1, 4, and 6 wk after experimental treatment. In all sampling occasions except in week 4, a blood drop (0.01 mL) was kept in a heparinized tube for determination of leukocyte profile by flow cytometry (see below). Treatment doses were established according to the manufacturer's recommendations and dosages used in previous studies (Saino et al., 2002).

We used hemagglutination inhibition (HI) to assess NDV antibody levels in sera. The HI test is a classic indirect diagnostic assay that is widely used and serves as a gold standard for detecting NDV antibodies in birds (Alexander, 2009). The concentration of NDV antibodies was determined by adding 4 hemagglutinin units of HIPRAVIAR®-CLON (E.Newcastle, clon CL/79) antigen to test sera sequentially diluted from 1∶2 to 1∶640. VLDIA053 (HAR-NDL, NDV strain La Sota) and VLDIA030 (SPF-CH-Chicken negative) were used as positive and negative controls, respectively. Eleven individuals with initial HI titers exceeding 1∶16 were removed from the analyses, as presumably these had been previously exposed to NDV in the wild. Leukocyte profiles were determined following Uchiyama et al. (2005) to estimate relative lymphocyte, granulocyte, and monocyte counts with the use of a flow cytometer (Guava Easy Cyte Plus, Guava Technologies, Hayward, California, USA). Variation in NDV antibody concentration (expressed as the inverse of the dilution factor) was analyzed as the dependent variable in a generalized linear mixed model with normally distributed error and identity link. Comparisons among factor levels were done with post hoc t-tests on least-squares means (Fig. 1). Because adjuvant and control treatments did not differ in any of the sessions (data not shown), data were merged into a single control group to improve statistical power. Treatment and session were included as main effects, with cage as a random factor and individual as repeated subject as implemented in proc GLIMMIX SAS 9.2. Additionally, the analyses were repeated with the leukocyte counts as covariates in the previous models, and later as dependent variables with treatment and session as main effects and cage as random factor. Degrees of freedom for fixed effects were estimated by means of the between–within approximation (SAS Institute Inc., 2009). Differences in sample sizes among treatments and sessions are due to a few individuals escaping, and insufficient sera for some laboratory analyses.

Sex (F_1,61 = 0.06; P = 0.81), the interaction between sex and treatment (F_3,58 = 0.72; P = 0.54), body mass (F_1,119 = 0.04; P = 0.84), tarsus (F_1,61 = 2.11; P = 0.15), and wing length (F_1,61 = 1.00; P = 0.32) were initially included as covariates and later removed from the models, as there was no significant effect on the dependent variable.

Concentration of NDV antibodies in response to vaccination varied by treatment (F_3,62 = 12.55; P<0.0001), and by experimental session (F_3,120 = 23.55; P<0.0001; Fig. 1). Individuals did not differ in their NDV antibody concentration among treatments at the beginning of the experiment (F_3,120 = 1.76; P = 0.16). However, as indicated by the significant interaction between treatment and session (F_9,120 = 2.91; P = 0.004) differences among treatments appeared after 1 wk (F_3,120 = 4.13; P = 0.008), reached a maximum at week 4 (F_3,120 = 16.27; P<0.001), and remained unchanged until week 6 (F_3,120 = 8.21; P<0.001; see Fig. 1 for post hoc tests).

Leukocyte profile changed throughout the experiment as the proportion of lymphocytes (F_2,103 = 4.84; P = 0.010) and monocytes (F_2,103 = 7.03; P = 0.001) declined over time, but the proportion of granulocytes increased (F_2,103 = 6.53; P = 0.002), independently of treatment (all P>0.5, see Table 1). Leukocyte profiles were not related to antibody concentration, either when included singly or together as covariates in the previously described models (all P>0.5).

As found in earlier studies on poultry, the inactivated NDV vaccine elicited a significant humoral response in wild sparrows, but did not affect cell-mediated immunity as measured by leukocyte profiles (Popovic et al., 2010). Different treatments did not affect the birds' serology when sampled at week 1 postchallenge, but a serologic response was detected and peaked after week 4, remaining at this level until the end of the experiment. The highest vaccine dose (0.2 mL) induced a positive response in week 1 after treatment, with titers over 1/16; this response remained significantly different from controls from week 4 until week 6, as found by Lloyd and Wernery (2008) in hybrid falcons. Both lower doses of vaccine (0.05 and 0.1 mL) followed the same pattern as described for the highest dose, but the effects were not detectable at week 1 after vaccination and titers remained close to 1/16, which is normally considered as the positive threshold for the diagnosis of exposure to NDV (Alexander, 2009).

We detected no cell-mediated response to experimental vaccination. This agrees with studies questioning the relevance of cell-mediated immunity in the adaptive response to NDV (Reynolds and Maraqa, 2000). All leukocyte profiles (lymphocytes, granulocytes, and monocytes) remained unaltered among treatments. However, leukocyte profiles from all birds changed as the experiment proceeded, most likely as a result of a response to captivity. The ratio of heterophils (i.e., granulocytes) to lymphocytes is related to physiologic stress (Müller et al., 2011; Johnstone et al., 2012). Inactivated NDV vaccine elicits a humoral response in wild House Sparrows with doses above 0.05 mL per individual, and this response was traced from week 4 to week 6 postchallenge. Our results confirm the validity of these in vivo immunologic methods, which can be used in evolutionary, veterinary, and ecoepidemiologic studies on wild fauna.

We are indebted to Maribel and Plácido from la Cañada de los Pájaros for graciously allowing us to work on their property. We thank Juan Luis Barroso and Cristina Pérez for their help during fieldwork. JB was funded by JAE/Doc and Juan de la Cierva postdoctoral grants by both the Spanish Ministry of Science and European Regional Development Fund (FEDER) program. This work was partially supported by projects RNM118, RNM157, and P07-RNM-02511 of the Junta de Andalucía and the Spanish Ministry of Science project (CGL2009-11445). HIPRA Laboratory kindly supplied NDV vaccine. All procedures were approved by the ethical committee of the institute and the environmental agency of the Spanish government (N/RefS.:G YB/AFR/CMM), and complied with the current Spanish laws.