1. Introduction
Caseous lymphadenitis (CL) is a contagious chronic bacterial disease of global relevance that affects small ruminants, horses, cattle, pigs, wildlife, and even humans. It is also known as pseudotuberculosis or "cheesy gland." The disease is caused by a gram-positive facultative intracellular parasite called
Corynebacterium pseudotuberculosis (
Cp). It belongs to the vitally important CMNR group encompassing
Corynebacterium, Mycobacterium, Nocardia, and Rhodococcus [
1,
2,
3]. It is often referred to as "iceberg disease" because it can cause a gradual decline in physical condition, thin ewe syndrome, fertility problems, behavioral changes, decreased milk and wool production, and poor carcass quality [
4,
5,
6,
7]. Unfortunately, the disease is challenging to eradicate due to difficulties in detecting subclinically affected animals, poor response to antibiotic treatment, and the ability of
C.pseudotuberculosis to persist in different environments while evading the host's immune system [
8,
9].
Sheep infected with
Cp experience a slow progression of encapsulated pyogranulomas, which eventually lead to the formation of larger abscesses in superficial or visceral lymph nodes and organs, such as the lungs, liver, and kidneys [
10,
11]. The main factors contributing to the virulence of
Cp are its cell wall lipids and exotoxin phospholipase D, which promote bacterial persistence and spread within the host and result in macrophage death and chronic inflammation [
12,
13,
14]. Researchers are currently conducting genomic and proteomic studies to identify Cp's antigenic potential and pathogenicity islands involved in evading the host's immune system. These studies aim to produce more effective vaccines for preventing infection and limiting emerging antibiotic resistance to reduce CL outbreaks on farms [
5,
15,
16,
17]. However, the cost of commercial vaccines remains high, and they have yet to provide full protection against infection or effective control of CL.
The reasons why some sheep and goats are more susceptible to
Cp than others, whether due to physiological or genetic factors, are still not fully understood. However, studies suggest that a cellular immune response, specifically a Th1 response, plays a significant role in controlling the infection, as the microorganism is facultatively intracellular. The formation of pyogranulomas, which are characteristic of
C. pseudotuberculosis infection, involves both humoral and cell-mediated immunity, a complex process that requires the activation of macrophages and T-lymphocytes [
18,
19,
20]. Recent advances in high-throughput genomic technologies have enabled the first comprehensive profiling of the goat spleen transcriptome and proteome in response to experimental infection with
Cp [
21,
22,
23]. Currently, genome-wise approaches and the generation of big data are considered promising selection-associated techniques that can provide animal farming industries with the ability to cope with the issues caused by diseases through breeding for health traits [
24].
Cases of CL have increased in the Czech Republic over the last decade, affecting an average of 20% of animals in approximately 30 tested sheep and goat flocks. To better understand the disease and host defence, we studied global gene expression in sheep infected with Cp. The study's main goals were the comprehensive characterization of the ovine whole blood transcriptome in the later course of the infection in naturally infected ewes. Moreover, we performed a functional comparison of the differentially expressed genes between healthy and affected animals based on their differing serological statuses. Thus, this research provides new insights into the host immune response against Cp in sheep.
4. Discussion
We first describe the potential influence of gene expression on difficult-to-treat caseous lymphadenitis through the analysis of the blood transcriptome in sheep. In contrast to previous animal model studies conducted under controlled experimental conditions in mice, goats, or cell cultures [
18,
31,
32,
33,
34], we measured gene expression in sheep raised under field conditions. This approach yielded unique insights into the host‒pathogen relationship but also has certain limits. Despite the comprehensive data, transcriptome analysis can only offer limited information about metabolic and health status during sampling. In addition, soluble immune molecules often act briefly, locally, at low concentrations and can therefore be challenging to analyse in the blood. It is important to note that whole blood is a complex mixture of various cells and may not necessarily reflect molecular events in organs [
35]. Nevertheless, the alteration of gene expression in peripheral blood may be specific with disease progression and used for pathophysiology identification [
36]. Understanding how the host and pathogen interact in the initial phases of a disease is crucial to comprehending the infection's mechanism. However, transcriptomic analysis of the later phases may guide changes occurring in the preclinical and clinical phases of the disease [
37].
In this study, we highlight the value of RNA-seq in monitoring genes that could modify the immune response during infection. Our findings show that the blood transcriptome differences between the two experimental groups were less pronounced. Several authors have observed that the full onset of CL can be preceded by an incubation period in which antibodies are not detectable, and some animals may not show them at any phase after the infection [
38,
39,
40]. There is also evidence of a slight difference in the blood transcriptome between infected
Mycobacterium avium subsp. paratuberculosis (MAP) and exposed animals belonging to paratuberculosis-positive herds [
37,
41]. The phylogenetically related genera
Mycobacterium and
Corynebacterium probably share some pathological features due to their intracellular nature [
4,
7].
A survey of DEGs observed in the experimental groups discussed below is presented in
Table 2,
Table 3 and
Table 4. Compared to the previous transcriptome measure of early exposure to
Cp in goats, [
21] the number of differentially expressed genes was substantially lower in our study. Additionally, the changes in metabolic processes and pathways in the later phase of infection were less pronounced. The host immune mechanisms appeared to be relatively balanced in controlling the adverse effects of infection, especially in diseased ewes. The number of differentially expressed genes in infected animals often decreases with time [
42].
A member of the Ig superfamily, VCAM-1 (vascular cell adhesion molecule), was identified as the main differentially expressed unique gene between diseased and exposed ewes. It is a cell surface glycoprotein expressed by the cytokine-activated endothelium that helps regulate inflammation-associated vascular adhesion and the transendothelial migration of macrophages and T cells. The expression of VCAM-1 is activated by proinflammatory cytokines and, together with integrins, it plays a central role in leukocyte recruitment during inflammation [
43]. Its high concentration in the blood of diseased animals may indicate active regulation of the inflammatory response to ongoing
Cp infection.
The most abundant and unique DE gene investigated in the EP group compared to healthy controls was KLRB1 (killer cell lectin-like receptor subfamily B member 1, CD161), which encodes a receptor expressed on natural killer (NK) cells and peripheral blood memory T cells. It defines a functionally distinct subset of proinflammatory NK cells and marks cells that have retained the ability to respond to innate cytokines during their differentiation. It is interesting to note that upregulation of KLRB1 was also found in whole blood of MAP-positive cattle [
44]. The most downregulated gene in diseased animals detected using RNA-seq was a common gene for microfibril-associated protein 5, MFAP5. In cattle, it is involved in definitive haemopoiesis, and its deficiency in mice is connected with decreased levels of neutrophils in circulation [
45].
The highly abundant transcript among exposed animals was hemicentin, HMCN1. This gene encodes a conserved large ECM protein, a member of the Ig superfamily [
46]. It is involved in cell adhesion, cytoskeleton organization and response to bacteria. On the other hand, mesenteric estrogen-dependent adipogenesis, MEDAG, was examined as the most downregulated transcript. This gene is believed to promote adipocyte differentiation, lipid accumulation, and glucose uptake in mature adipocytes, but its role in immunity is yet to be determined [
47]. It is also noteworthy that the transcriptional activity of the PRG4 gene was deficient in the EN group. It has been proposed that PRG4 is regulated by the inflammatory response, typically in a negative fashion, and once inflammation has subsided, PRG4 returns to homeostatic levels to maintain tissue health [
48].
The infection primarily interfered with the expression of many sensing surface receptors. Various other differentially expressed C-lectin receptors of NK cells and macrophages were identified, such as CLECL1 and CLEC2D and CLEC4E (Mincle). Recently, it was discovered that Mincle (macrophage inducible Ca2+-dependent lectin) acted as a receptor for corynomycolic acids of the corynebacterial cell wall. It probably cooperates with other C-lectin receptors in
Cp recognition, activation of immune cells and modulation of the proinflammatory response to prevent possible tissue damage [
49,
50,
51]. In addition to the C-lectin receptors, RNA-Seq analysis identified several G protein-coupled receptors and six different transcripts of LIR receptors. Ig-like receptors LIR, LILRA5 and LILRA6 of NK cells showed DE in both experimental groups, which was more pronounced in exposed ewes. They drive leukocyte activity and play an essential role in regulating the immune response. The significance of LIR receptors in the innate immune response has been recently broadened to include the ability to develop antigen-specific immune memory [
52].
The category of peptide presenting antigens to NK receptors was almost highly upregulated in affected ewes. It consists of two classical BOLA class I histocompatibility antigens, OVAR and OLA-I, and MHC-III lymphocyte antigen 6 family member G6C, LY6G6C, and nonclassical MHC proteins and cooperating molecules, MHC class I polypeptide-related sequence B, MICB, UL16-binding protein 2-like, ULBP2 and butyrophilin subfamily 2, member A2, BTN2A2. Many studies have confirmed the pivotal role of MHC molecules in antigen presentation required to elicit an immune response against invading pathogens. The expression of MHC genes is altered during mycobacterial infection in cattle [
42,
53,
54]. In transcriptome analysis of the
Cp-infected spleen of dairy goats, the MHC class I protein complex was identified as one of the most enriched terms [
21]. The genes for CD1A and CD1E molecules structurally related to MHC, which present lipids of microbial origin to T-cell receptors, were differentially expressed only in the transcriptome of the diseased ewes. The CD1 system is considered a versatile player in the immune response, sitting at the crossroads of innate and adaptive immunity, which may be involved in numerous infectious and inflammatory responses [
55].
Our study found that both experimental groups had a decreased abundance of GZMA, GZMB, and GZMH transcripts in their blood. These transcripts are part of the granzyme serine protease family, which is specific to cytolytic T lymphocytes and NK cells. Proteases directly contribute to immune defence by inducing cell cytolysis and pathogen clearance in innate immunity [
56]. GZMA is a potential biomarker of human
Mycobacterium tuberculosis (MT) infection and disease, as its serum levels are significantly lower in patients with active MT disease [
57]. Additionally, GZMA may be captured and internalized within mycobacteria-infected monocytes in influenced organs to inhibit bacterial growth and prevent infection [
58]. Based on our findings, we hypothesize that granzyme levels may fluctuate in the peripheral blood of CL-affected animals and that its lower transcription can be attributed to a particular disease stage. The family of GIMAP enzymes (specifically GIMAP1, GIMAP7, and GIMAP8) was expressed in either the same or different directions in both diseased and exposed sheep. GTPases of immunity-associated proteins are regulators of lymphocyte survival and homeostasis and are linked to inflammatory and autoimmune diseases [
59,
60]. Recent studies suggest that GIMAP proteins may interact with each other and be involved in the movement of the cellular cargo along the cytoskeletal network [
61].
Several authors have emphasized the significance of the cellular immune response and proinflammatory cytokines in limiting bacterial growth and dissemination inside the host in the initial phase of
Cp infection [
18,
62]. Through transcriptome analysis of ovine blood samples, we confirmed the involvement of T lymphocyte-mediated immunity. Gamma-delta T-cell activation predominates in exposed ewes, along with the expression of their specific WC1.1 antigen. γδ T lymphocytes are thought to play crucial roles in immunosurveillance and host defence, particularly against mycobacteria [
63]. Glycoprotein WC1.1 is highly represented in ovine peripheral blood and acts as both a γδ T-cell activating coreceptor and pattern recognition receptor [
64]. In goat cell culture, only WC1(+) γδ T cells were able to produce proinflammatory cytokines [
65]. We also observed increased regulation of innate proinflammatory NF-kappa B signaling in EN group. In contrast, a detailed analysis of the goat transcriptome revealed downregulation of the NF-κB pathway in
Cp-infected individuals [
21]. However, bovine MAP-infected macrophages produce larger amounts of NF-κB during the initial phase of infection [
66]. Many DEGs (approximately 30) in exposed animals are involved in programmed cell death and response to cellular stress as part of normal physiological processes and native immunity. The regulation of their expression tends to be balanced, probably to maintain homeostasis during infection and to prevent excessive cell damage. It has been reported that apoptosis is an essential defence mechanism for host resistance to pathogen invasion. However,
Cp infection likely contributes to elevated programmed cell death by apoptosis [
21]. Accordingly, death of host cells colonized by corynebacteria has been observed to occur more frequently during the early stage of the pathogenic process [
67].
In our study of positive sheep with caseous lymphadenitis, we observed an increase in the regulation of the adaptive immune response through signal transduction and cytokine signalling. The pathogenesis of ovine caseous lymphadenitis was shown to be associated with the production of cytokines at the pyogranuloma level. However, the local cytokine patterns associated with different courses of infection and the time following infection were not distinguished [
2,
34]. We found both positively and negatively regulated host responses to cytokines but partly lowered cytokine and chemokine production and its regulation. During this phase of the disease, lower expression of chemokine CXCL10, chemokine receptor CXCR6, complement protein C4BPA, complement receptor C3AR1, and anti-inflammatory interleukin IL13 suggests a limited inflammatory response, complement activation, chemotactic activity and lymphocyte migration towards the inflammation site. Likewise, MAP has the ability to downregulate the host complement, hence increasing the chance of survival of bacteria when outside the host cells [
53]. Recently, C-X-C motif chemokine ligand 10 has been proposed as a possible biomarker of bovine tuberculosis with the potential to differentiate between latent and active disease. Due to the granulomatous nature of mycobacteria and also related corynebacteria, chemokine recruitment of leukocytes may be a host response to contain invading bacteria, and the restriction of this process by bacteria can subvert the host immune response and establish a latent infection [
54]. We suppose that the mechanism behind this phenomenon may be the ability of phospholipase D to reduce leukocyte migration towards sheep serum [
68].
Although the transcriptional regulation of the humoral IR appeared to decrease, B-cell activation and B-cell receptor signalling orchestrated by the highly expressed receptor TNFRSF13C (TNF receptor superfamily member 13C) were upregulated in diseased ewes. This receptor is considered the principal receptor required for B-cell-activating factor (BAFF)-mediated mature B-cell survival [
69]. The initial immune response to
Cp has been shown to include a strong humoral component, but little is known about the kinetics of protective antibodies [
62,
70]. After experimental infection in goats, the humoral IR showed individual intensity variations and regularly declined over time [
20]. The positive regulation of the adaptive immune response based on somatic recombination of immune receptors built from Ig superfamily domains indicates the potential of the host to develop both clonal-specific antibodies against
Cp and protective immunological memory during disease progression. We suggest that this can contribute to more effectively eliminating bacteria that can escape from granulomas and spread through the lymphatic system.
We identified significant enrichment of several pathways that may be altered in relation to ongoing infection. In diseased animals, the only statistically significant pathway of Binding and Uptake of Ligands by Scavenger Receptors was recognized. Scavenger receptors bind free extracellular ligands as the initial step in their clearance from the body. They represent an important part of the innate immune defence by acting as bacterial pattern-recognition receptors. The receptors can modulate the inflammatory response as coreceptors of Toll-like receptors, and some intracellular pathogens utilize them to enter host cells [
71]. Pathways of ECM proteoglycans and ECM organization containing significant amounts of DEGs were also highly differentially regulated in our research, especially in a negative way. ECM, as a major component of the cellular microenvironment, influences cell adhesion and migration, and its composition is highly heterogeneous and dynamic, being constantly remodelled [
72]. Therefore, we suppose the current infection can degrade the essential immune and adhesive functions of ECM and its large protein structures. In accordance with our results, transcriptional profiling of Holstein cows yielded similar findings on ECM modifications in response to natural MAP infection [
73].
We also observed substantial expression differences in the immunoregulatory interactions between lymphoid and nonlymphoid cell pathways, which play a critical role in modifying the response of lymphoid cells to pathogenic organisms. Lymphoid cells are able to regulate their location and movement in accordance to their state of activation, and home in on tissues expressing the appropriate complementary ligands [
74]. The pathway involves mainly identified NK immune receptors (LILRs and KIRs) and adhesion molecules (i.e., VCAM-1) necessary for antigen presentation by MHC and cooperating molecules (i.e., MICB) and maintaining immunological synapses [
75]. It is transcriptionally differentially regulated in both experimental groups facing infection compared to healthy animals.
The p53 pathway was found to be the only one significantly affected in exposed ewes. This finding aligns with the significant transcriptional regulation of genes related to cell death and cell cycle. The p53 transcription factor, known as the tumour suppressor protein, is the key gatekeeper in the cellular response to stress signals. It can help exposed animals respond adequately to stress and prevent DNA damage that is potentially caused by infectious pressure. It also allows cells to recover from damage and survive, preventing premature cell death.
In conclusion, we want to draw attention to some of the limitations or shortcomings of this study that need to be considered. First, to improve the accuracy of the results, it may be necessary to work with a larger sample size during CL infection and exposure. Additionally, due to the incomplete functional annotation of sheep genes, it was necessary to rely on bovine or human homologues, which prevented full interpretation of some discovered genes. Last, the differences in gene expression levels we observed between the investigated ewe groups could be affected by other factors, such as the animal's physiological status, management, or environmental differences.