Presence of dendritic cells in chicken spleen cell preparations and their functional interaction with the parasite Toxoplasma gondii

Targeting Haemagglutinin Antigen of Avian Influenza Virus to Chicken Immune Cell Receptors Dec205 and CD11c Induces Differential Immune-Potentiating Responses

Improving the immunogenicity and protective efficacy of vaccines is critical to reducing disease impacts. One strategy used to enhance the immunogenicity of vaccines is the selective delivery of protective antigens to the antigen presenting cells (APCs). In this study, we have developed a targeted antigen delivery vaccine (TADV) system by recombinantly fusing the ectodomain of hemagglutinin (HA) antigen of H9N2 influenza A virus to single chain fragment variable (scFv) antibodies specific for the receptors expressed on chicken APCs; Dec205 and CD11c. Vaccination of chickens with TADV containing recombinant H9HA Foldon-Dec205 scFv or H9HA Foldon-CD11c scFv proteins elicited faster (as early as day 6 post primary vaccination) and higher anti-H9HA IgM and IgY, haemagglutination inhibition, and virus neutralisation antibodies compared to the untargeted H9HA protein. Comparatively, CD11c scFv conjugated H9HA protein showed higher immunogenic potency compared to Dec205 scFv conjugated H9HA protein. The higher immune potentiating ability of CD11c scFv was also reflected in ex-vivo chicken splenocyte stimulation assay, whereby H9HA Foldon-CD11c scFv induced higher levels of cytokines (IFNγ, IL6, IL1β, and IL4) compared to H9HA Foldon-Dec205 scFv. Overall, the results conclude that TADV could be a better alternative to the currently available inactivated virus vaccines.

Experimental infection with Toxoplasma gondii in broiler chickens (Gallus domesticus): seroconversion, tissue cyst distribution, and prophylaxis

Toxoplasma gondii is a widespread zoonotic protozoan that infects most species of mammals and birds, including poultry. This study aimed to investigate the course of T. gondii infection and the efficacy of diclazuril and Artemisia annua in preventing infection in experimentally infected chickens. Seventy-five 1-month-old chickens, female and male, were randomly divided into five groups (n = 15 each) as follows: (1) uninfected untreated (negative control, NC); (2) infected with T. gondii genotype II/III isolated from a wild cat (group WC); (3) infected with T. gondii genotype II isolated from a domestic cat (group DC); (4) infected with T. gondii domestic cat strain and treated with the anticoccidial diclazuril (group DC-D); and (5) infected with T. gondii domestic cat strain and treated with the medicinal plant Artemisia annua (group DC-A). Clinical signs, body temperature, mortality rate, weight gain, feed conversion ratio, hematological parameters, and the presence of T. gondii-specific IgY antibodies were recorded in all groups. Five chickens per group were euthanized 28 days post-infection (p.i.) and their brains, hearts, and breast muscle tested for T. gondii by mouse bioassay and polymerase chain reaction (PCR). No clinical signs related to the experimental infection were observed throughout the study period. T. gondii-specific antibodies were detected by day 28 p.i., but not in all infected chickens. Overall, T. gondii DNA was detected (bioassay or tissue digests) in all infected and untreated chickens (10/10), while viable parasite (bioassay) was isolated from 7 out of 10 chickens. The parasite was most frequently identified in the brain (7/10). There were no differences in the T. gondii strains regarding clinical infection and the rate of T. gondii detection in tissues. However, higher antibody titers were obtained in chickens infected with T. gondii WC strain (1:192) comparing with T. gondii DC strain (1:48). A. annua reduced replication of the parasite in 3 out of 5 chickens, while diclazuril did not. In conclusion, broiler chickens were resistant to clinical toxoplasmosis, irrespective of the strain (domestic or wild cat strain). The herb A. annua presented prophylactic efficacy by reduced parasite replication. However, further studies are required aiming at the efficacy of diclazuril and A. annua for the prevention of T. gondii infection in chickens using quantitative analysis methods.

A Comparison of Transcriptional Diversity of Swine Macrophages Infected With TgHB1 Strain of Toxoplasma gondii Isolated in China

Toxoplasma gondii is an apicomplexan parasite infecting human and animals, causing huge public health concerns and economic losses. Swine alveolar macrophage plays an important role in controlling T. gondii infection. However, the mechanism by which macrophages infected with T. gondii function in the immunity to the infection is unclear, especially for local isolates such as TgHB1 isolated in China. RNA-seq as a valuable tool was applied to simultaneously analyze transcriptional changes of pig alveolar macrophages infected with TgRH (typeI), TgME49 (typeII) or TgHB1 at different time points post infection (6, 12, and 24 h). Paired-end clean reads were aligned to the Sscrofa10.2 pig genome and T. gondii ME49 genome. The differentially expressed genes of macrophages and T. gondii were enriched through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes, respectively. Compared to the TgRH and TgME49 infection groups, 307 down-regulated macrophage genes (mainly enriched for development and metabolism) and 419 up-regulated genes (mainly enriched for immune pathways) were uniquely expressed in the TgHB1 infection group. Additionally, 557 down-regulated and 674 up-regulated T. gondii genes (mainly enriched in metabolism and biosynthesis) were uniquely expressed in the TgHB1 infection group. For validation purposes, some of the differentially expressed genes of macrophages involved in immune-related signaling pathways were used for further analysis via real time quantitative reverse-transcription polymerase-chain reaction (qRT-PCR). This work provides important insights into the temporal immune responses of swine alveolar macrophages to infection by the strain TgHB1 isolated from China, and is helpful for better understanding of the T. gondii genotype-associated activation of macrophages during early phase of the infection.

CD11d is a novel antigen on chicken leukocytes

In life sciences, antibodies are among the most commonly used tools for identifying, tracking, quantifying and isolating molecules, mainly proteins. However, it has recently become clear that antibodies often fall short with respect to specificity and selectivity and in many cases target proteins are not even known. When commercial availability of antibodies is scarce, e.g. for targeting proteins from farm animals, researchers face additional challenges: they often have to rely on cross-reactive antibodies, which are poorly characterized for their exact target, their actual cross-reactivity and the desired application. In this study, we aimed at identifying the true target of mouse monoclonal antibody 8F2, which was generated against chicken PBMC and used for decades in research, while it's actual target molecule remained unknown. We used 8F2 antibody for immunoprecipitation in chicken PBMC and subsequently identified its true target as CD11d, which was never described in chicken lymphocytes before, by quantitative LC-MSMS. The most abundant interactor of CD11d was identified as integrin beta 2. The existence of this alpha integrin was therefore clearly proven on protein level and provides a first basis to further assess the role of CD11d in chickens in future studies. Data are available via ProteomeXchange with identifier PXD017248. SIGNIFICANCE: Our studies determined CD11d as the true target of a previously uncharacterized mouse monoclonal antibody 8F2, generated against chicken peripheral blood derived mononuclear cells (PBMC). This is therefore now first member of alpha integrins in chickens, that existence was now clearly identified on protein level. The additional identification of CD11d interactors provides information on integrin-dependent regulation of signaling networks, allowing further functional studies.

Characterization of splenic MRC1hiMHCIIlo and MRC1loMHCIIhi cells from the monocyte/macrophage lineage of White Leghorn chickens

Monocytes/macrophages, which are found in a variety of organs, maintain tissue homeostasis at a steady state and act as the first line of defence during pathogen-induced inflammation in the host. Most monocyte/macrophage lineage studies in chickens have been largely performed using cell lines, while few studies using primary cells have been conducted. In the present study, the phenotypic and functional characteristics of splenic monocyte/macrophage lineage cells during steady state and inflammatory conditions were examined. Splenic monocyte/macrophage lineage cells could be identified as MRC1^loMHCII^hi and MRC1^hiMHCII^lo cells based on their surface expression of MRC1 and MHCII. In the steady state, MRC1^loMHCII^hi cells were more frequently found among MRC1⁺ cells. MRC1^loMHCII^hi cells expressed a higher number of antigen-presenting molecules (MHCII, MHCI, and CD80) than MRC1^hiMHCII^lo cells. In contrast, MRC1^hiMHCII^lo cells showed better phagocytic and CCR5-dependent migratory properties than MRC1^loMHCII^hi cells. Furthermore, MRC1^hiMHCII^lo cells infiltrated the spleen in vivo and then became MRC1^loMHCII^hi cells. During lipopolysaccharide (LPS)-induced inflammatory conditions that were produced via intraperitoneal (i.p.) injection, the proportion and absolute number of MRC1^hiMHCII^lo cells were increased in the spleen. Uniquely, inflammation induced the downregulation of MHCII expression in MRC1^hiMHCII^lo cells. The major source of inflammatory cytokines (IL-1β, IL-6, and IL-12) was MRC1^loMHCII^hi cells. Furthermore, MRC1^hiMHCII^lo cells showed greater bactericidal activity than MRC1^loMHCII^hi cells during LPS-induced inflammation. Collectively, these results suggest that two subsets of monocyte/macrophage lineage cells exist in the chicken spleen that have functional differences.

Current knowledge about interactions between avian dendritic cells and poultry pathogens

In poultry production conditions today, birds are surrounded by viral, bacterial, and parasitic agents. DCs are the main antigen-presenting cells located in tissues of the body, and their role involves recognizing antigen structures, engulfing and processing them, and subsequently presenting antigen peptides on their surface by major histocompatibility complex, where T cells and B cells are stimulated and can begin appropriate cellular and antibody immune response. This unique function indicates that these cells can be used in producing vaccines, but first it is necessary to culture DCs in vitro to identify the principles of their interactions with pathogens. The following review summarizes our current knowledge about avian dendritic cells and their interactions with pathogens. It provides a basis for future studies of these unique cells and their use in vaccine development.

Development of innate immunity in chicken embryos and newly hatched chicks: a disease control perspective

Newly hatched chickens are confronted by a wide array of pathogenic microbes because their adaptive immune defences have limited capabilities to control these pathogens. In such circumstances, and within this age group, innate responses provide a degree of protection. Moreover, as the adaptive immune system is relatively naïve to foreign antigens, synergy with innate defences is critical. This review presents knowledge on the ontogeny of innate immunity in chickens pre-hatch and early post-hatch and provides insights into possible interventions to modulate innate responses early in the life of the bird. As in other vertebrate species, the chicken innate immune system which include cellular mediators, cytokine and chemokine repertoires and molecules involved in antigen detection, develop early in life. Comparison of innate immune systems in newly hatched chickens and mature birds has revealed differences in magnitude and quality, but responses in younger chickens can be boosted using innate immune system modulators. Functional expression of pattern recognition receptors and several defence molecules by innate immune system cells of embryos and newly hatched chicks suggests that innate responses can be modulated at this stage of development to combat pathogens. Improved understanding of innate immune system ontogeny and functionality in chickens is critical for the implementation of sound and safe interventions to provide long-term protection against pathogens. Next-generation tools for studying genetic and epigenetic regulation of genes, functional metagenomics and gene knockouts can be used in the future to explore and dissect the contributions of signalling pathways of innate immunity and to devise more efficacious disease control strategies.

CD205-positive, Sepharose-induced peritoneal exudate cells: a new resource for DC research in the chicken

Dendritic cells (DC) are important antigen-presenting cells and are among the least characterized immune cells in the chicken. In order to obtain chicken DC, current protocols require isolation of bone marrow myeloid progenitor cells and induction of DC differentiation with supplemental cytokines or negative selection of splenic cell preparations. Chicken peritoneal exudate cells (PEC) have traditionally been a source of various immune cells for ex vivo studies, primarily to investigate heterophils and macrophages. In this study, we observe the presence of CD205⁺ PEC populations, a marker of DC, as an additional resource to isolate and study chicken primary DCs. A panel of monoclonal antibodies was developed against the chicken CD205 DC marker and used to isolate CD205⁺ DC from the PEC population using magnetic bead cell sorting. This study reports the development of new anti-CD205 monoclonal antibodies as a reagent for chicken DC research, as well as PEC as a potential source of CD205⁺ DC for ex vivo studies in the chicken.

Host-pathogen interaction in Toxoplasma gondii-infected mixed chicken blood cell cultures

Toxoplasma gondii has the ability to infect various nucleated cell types in different hosts. The aim of the present study was to investigate which chicken blood cells were targeted by T. gondii in a mixed blood cell culture similar to in vivo conditions and to evaluate parasite-host cell interactions. The study consisted of two subsequent experiments. In experiment 1, we applied T. gondii tachyzoites (ME49) at a multiplicity of infection of 1 tachyzoite per blood cell and examined parasite replication, cytokine, and inducible nitric oxide synthase (iNOS) mRNA expression between 1 h and 48 h post-infection (p.i.) by quantitative PCR. By using T. gondii RH-GFP tachyzoites expressing the green fluorescent protein (GFP) in experiment 2, we aimed for visualizing infected cells by confocal laser scanning microscopy (CLSM) and flow cytometric analysis at 24 h p.i. The parasite replication curve showed a massive decrease of parasite stages until 24 h p.i. followed by an approximately plateau phase. We observed mainly significantly increased iNOS mRNA expression levels in T. gondii-infected culture compared to uninfected cells. Flow cytometry and CLSM data confirmed monocytes/macrophages as main target cells for T. gondii. Moreover, different lymphocytes like B cells and cytotoxic T cells seem to be targeted to a low extent. Our findings indicate that monocytes/macrophages play a key role during T. gondii infection in chicken as host cells and triggering of immune response. To the best of our knowledge, this is the first report of a mixed chicken blood cell culture experimentally infected with T. gondii.

Mutual interactions of the apicomplexan parasites Toxoplasma gondii and Eimeria tenella with cultured poultry macrophages

BACKGROUND

Toxoplasma gondii and Eimeria tenella are two common parasites in poultry. Mixed infections are likely to occur frequently in chickens due to the high prevalence of both pathogens. In this study, we investigate the co-occurrence of the two pathogens in the same immunocompetent host cell population towards potential parasite-parasite as well as altered patterns of parasite-host interactions.

METHODS

Primary macrophages from chicken blood were co-infected in vitro with T. gondii tachyzoites (RH strain) and E. tenella sporozoites (Houghton strain) for 72 h. Morphological observations by light microscopy and assessments of parasite replication by quantitative real-time PCR (qPCR) were performed at 24, 48 and 72 h post-infection (hpi). Six host cell immune factors previously linked to T. gondii or E. tenella infection were selected for gene expression analysis in this study.

RESULTS

Distinct morphological changes of macrophages were observed during mixed infection at 24 hpi and immunological activation of host cells was obvious. Macrophage mRNA expression for iNOS at 48 hpi and for TNF-α at 72 hpi were significantly elevated after mixed infection. Distinct upregulation of IL-10 was also present during co-infection compared to Eimeria mono-infection at 48 and 72 hpi. At 72 hpi, the total number of macrophages as well as the number of both parasites decreased markedly. As measured by qPCR, E. tenella population tended to increase during T. gondii co-infection, while T. gondii replication was not distinctly altered.

CONCLUSIONS

Mutual interactions of T. gondii and E. tenella were observed in the selected co-infection model. The interactions are supposed to be indirect considering the observed changes in host cell metabolism. This study would thus help understanding the course of co-infection in chickens that may be relevant in terms of veterinary and zoonotic considerations.

Characterisation of susceptibility of chicken macrophages to infection with Toxoplasma gondii of type II and III strains

Toxoplasma gondii is known to be able to infect any nucleated cell including immune cells like macrophages. In addition, it is assumed that macrophages serve as trojan horse during distribution in hosts. The underlying causes of parasite host interaction remain yet not fully understood. The aim of the present study was to investigate susceptibility of chicken macrophages to infection with T. gondii and the process of infection in avian cells in comparison to cells of mammalian origin. Primary avian blood monocyte-derived macrophages were infected with tachyzoites of type II (ME49) and III (NED) strains. Long term observations of parasite replication in primary macrophages were compared to data obtained in an avian macrophage cell line (HD11) and a standard cultivation mammalian cell line (VERO). Furthermore, we assessed the immune response of the primary macrophages by long-term investigation of gene expression of IL-1 beta, IL-12p40, Lipopolysaccharide induced TNF-alpha factor (LITAF) and inducible nitric oxide synthase (iNOS) comparing viable and heat-inactivated tachyzoites of the ME49 strain. Albeit, we found no differences between both strains, replication of tachyzoites in avian primary macrophages was significantly different from immortalized cell lines HD11 and VERO. The crucial period of parasite replication was between 8 and 24 h post-infection coinciding with the upregulation of gene expression of cytokines and iNOS revealing an active macrophage response at this period. Gene expression in macrophages was higher after infection with viable tachyzoites than by exposure of cells to heat-inactivated tachyzoites. Hence, we conclude that the process of penetration is pivotal for host cell response to the parasite both in avian as in mammalian cells.

Experimental Toxoplasma gondii and Eimeria tenella co-infection in chickens

The widespread apicomplexan parasites Toxoplasma gondii (T. gondii) and Eimeria tenella (E. tenella) are important pathogens with high prevalence in poultry. The aim of our study was the investigation of mutual influences in co-infected chickens, focusing on immune response and course of infection. Two separate trials were performed using in total 96 1-day-old chickens, divided into four study groups: group NC (negative control, uninfected), group PC-T (oral or intramuscular infection with T. gondii oocysts (trial 1) or tachyzoites (trial 2), respectively), group PC-E (oral infection with E. tenella (trial 1) or E. tenella and Eimeria acervulina (trial 2)), and group TE (co-infection). T. gondii and Eimeria infections were validated by different parameters, and cytokine expression in the gut and spleen was investigated. T. gondii-specific antibodies were detected earliest 4 days post infection (p.i.) by immunoblot and direct DNA detection was possible in 22.1% of all tissue samples from infected chickens. Eimeria spp. merogony seemed to be enhanced by co-infection with T. gondii, interestingly without marked differences in oocyst excretion between co-infected and Eimeria spp. mono-infected chickens. An increase of messenger RNA (mRNA) expression of Th1- (IFN-γ, IL-12, TNF-α) and Th2-related cytokines (IL-10) mainly in groups PC-E and TE was observed, however, without statistically significant differences between co-infection and single infection with Eimeria. In conclusion, most of the measurable immune response could be attributed to Eimeria infection. To the best of our knowledge, this is the first report on co-infection experiments of T. gondii with Eimeria spp. in chickens.

Characterization of Chicken Splenic-Derived Dendritic Cells Following Vaccine and Very Virulent Strains of Infectious Bursal Disease Virus Infection

Studies have shown that infectious bursal disease virus (IBDV) infects lymphoid cells, mainly B cells and macrophages. This study was aimed to examine the involvement of chicken splenic-derived dendritic cells (ch-sDCs) in specific-pathogen-free chickens following inoculation with IBDV vaccine strain (D78) and a very virulent (vv) strain (UPM0081). Following IBDV infection, enriched activated ch-sDCs were collected by using the negative selection method and were examined based on morphology and immunophenotyping to confirm the isolation method for dendritic cells (DCs). The presence of IBDV on enriched activated ch-sDCs was analyzed based on the immunofluorescence antibody test (IFAT), flow cytometry, and quantitative real-time PCR (RT-qPCR) while the mRNAs of several cytokines were detected using RT-qPCR. The isolated ch-sDCs resembled typical DC morphologies found in mammals by having a veiled shape and they grew in clusters. Meanwhile, the expression of DC maturation markers, namely CD86 and MHCII, were increased at day 2 and day 3 following vvIBDV and vaccine strain inoculation, respectively, ranging from 10% to 40% compared to the control at 2.55% (P < 0.05). At day 3 postinfection, IBDV VP3 proteins colocalized with CD86 were readily detected via IFAT and flow cytometry in both vaccine and vvIBDV strains. In addition, enriched activated ch-sDCs were also detected as positive based on the VP4 gene by RT-qPCR; however, a higher viral load was detected on vvIBDV compared to the vaccine group. Infection with vaccine and vvIBDV strains induced the enriched activated ch-sDCs to produce proinflammatory cytokines and Th1-like cytokines from day 3 onward; however, the expressions were higher in the vvIBDV group (P < 0.05). These data collectively suggest that enriched activated ch-sDCs were permissive to IBDV infection and produced a strong inflammatory and Th1-like cytokine response following vvIBDV infection as compared to the vaccine strain.

Replication of Toxoplasma gondii in chicken erythrocytes and thrombocytes compared to macrophages

Toxoplasma (T.) gondii is able to infect various cell types in different hosts. The replication of this parasite within different peripheral mononuclear blood cell populations in chicken has not yet been fully understood. Aim of the present study was to investigate the impact of chicken erythrocytes and thrombocytes as potential host cells for T. gondii. Cultures of primary avian erythrocytes and thrombocytes were inoculated with tachyzoites of T. gondii type II strain ME49. Parasite replication was detected by a quantitative real-time PCR at different times postinoculation until 24 or 48 h, respectively, displaying long-term investigations for the chosen cultures. The parasite replication curve showed a continuous decrease of parasite stages in erythrocytes and thrombocytes. Observations by light microscopy showed massive destruction for both cell populations. Few macrophages in between the infected thrombocytes were viable during the investigation period and showed internalised tachyzoites by confocal laser scanning microscopy. These findings show that T. gondii is not capable of replication in chicken erythrocytes and thrombocytes; therefore, both cannot be considered as potential host cells. In further consequence, monocyte-derived macrophages seem to be the key to the dissemination mechanisms for T. gondii in chicken.

Avian dendritic cells: Phenotype and ontogeny in lymphoid organs

Dendritic cells (DC) are critically important accessory cells in the innate and adaptive immune systems. Avian DCs were originally identified in primary and secondary lymphoid organs by their typical morphology, displaying long cell processes with cytoplasmic granules. Several subtypes are known. Bursal secretory dendritic cells (BSDC) are elongated cells which express vimentin intermediate filaments, MHC II molecules, macrophage colony-stimulating factor 1 receptor (CSF1R), and produce 74.3+ secretory granules. Avian follicular dendritic cells (FDC) highly resemble BSDC, express the CD83, 74.3 and CSF1R molecules, and present antigen in germinal centers. Thymic dendritic cells (TDC), which express 74.3 and CD83, are concentrated in thymic medulla while interdigitating DC are found in T cell-rich areas of secondary lymphoid organs. Avian Langerhans cells are a specialized 74.3-/MHC II+ cell population found in stratified squamous epithelium and are capable of differentiating into 74.3+ migratory DCs. During organogenesis hematopoietic precursors of DC colonize the developing lymphoid organ primordia prior to immigration of lymphoid precursor cells. This review summarizes our current understanding of the ontogeny, cytoarchitecture, and immunophenotype of avian DC, and offers an antibody panel for the in vitro and in vivo identification of these heterogeneous cell types.

Existence of conventional dendritic cells in Gallus gallus revealed by comparative gene expression profiling

The existence of conventional dendritic cells (cDCs) has not yet been demonstrated outside mammals. In this article, we identified bona fide cDCs in chicken spleen. Comparative profiling of global and of immune response gene expression, morphology, and T cell activation properties show that cDCs and macrophages (MPs) exist as distinct mononuclear phagocytes in the chicken, resembling their human and mouse cell counterparts. With computational analysis, core gene expression signatures for cDCs, MPs, and T and B cells across the chicken, human, and mouse were established, which will facilitate the identification of these subsets in other vertebrates. Overall, this study, by extending the newly uncovered cDC and MP paradigm to the chicken, suggests that these two phagocyte lineages were already in place in the common ancestor of reptiles (including birds) and mammals in evolution. It opens avenues for the design of new vaccines and nutraceuticals that are mandatory for the sustained supply of poultry products in the expanding human population.

Long-term investigations on Toxoplasma gondii-infected primary chicken macrophages

Toxoplasma (T.) gondii is known to infect various cell types including macrophages. In the present study, we generated monocyte-derived macrophage cultures from chicken blood. By flow cytometrical analysis, 84.5% of the cultivated cells showed typical macrophage properties. Macrophage cultures were cultivated at either 37 °C or 40 °C, respectively, and were infected 72 to 96 h post isolationem with tachyzoites of the T. gondii type II strain ME49 at a rate of 7.5 tachyzoites per host cell. Light microscopical investigations revealed incorporation of tachyzoites into the macrophages and gradual destruction of the infected macrophage culture. Parasite multiplication was observed by a quantitative real time PCR (qPCR) based on the 529-bp fragment specific for T. gondii. Samples were drawn 1 h post infectionem (p.i.), as well as 12, 24, 36, 48, and 72 h p.i. The parasite replication curve showed a transient decrease of parasite stages 12 h p.i. followed by a tachyzoite multiplication. The comparison of different culture conditions showed a significantly higher replication rate of T. gondii at 37 °C (median value 48 h p.i., 289.2% of the initial tachyzoite number) compared to cultures incubated at 40 °C (median value 48 h p.i., 73.1% of the initial tachyzoite number) throughout the observation period (P < 0.05). In general, replication rates were significantly lower than in a standard VERO cell cultures at 37 °C (P < 0.05). The observed differences were attributed to the physiological chicken macrophage reaction at 40 °C probably approximating the situation in vivo.