**4.1 Most intracellular pathogens induce either a Th1 or Th0 response in cattle**

During pathogen invasion, the host mounts a CD4+ T cell response that may or may not be effective enough to clear the infection. In humans, ineffective CD4+ T cell responses are associated with increased pathogenesis and progression towards chronic infections [153]. Cattle mostly launch either Th1 or Th0 responses against intracellular pathogens [154–157]. However, some bovine pathogens are able to establish chronic infections, which is possibly associated with ineffective CD4+ T cell responses [128, 158].

As observed in mice and humans, bovine Th1 responses are considered to be protective against diseases caused by intracellular pathogens such as *Theileria annulata*, and *Anaplasma marginale* [154, 155]. Indeed, researchers in the late 80s found that transferring serum from an immune animal into animals infected with theileriosis was not effective at controlling infection [159]. Several groups later discovered that CD8+ T cell responses but not humoral responses were effective at controlling disease, since antigen-specific CD8+ T cells from recovered animals demonstrated effective cytotoxicity to the autologous infected cells *in vitro* [160–162]. Further research revealed that *in vitro* activation of T cells with *Theileria*-infected macrophages predominantly induced IFNγ expression [163]. Similar to theileriosis, Th1 responses were also protective against *Anaplasma marginale* [155, 164]. In both infected and vaccinated animals, circulatory IFNγ levels were higher relative to their healthy counterparts [155, 164]. Similarly, IgG2 was increased in cattle infected with *Anaplasma marginale* [165]. Collectively, in both theileriosis and anaplasmosis, hosts seem to induce effective Th1 responses.

Bovine pathogens such as *Mycobacterium tuberculosis* and *Mycobacterium paratuberculosis* can shift a Th1-dominant response towards a Th0- or a Th2-dominant response as the infection progressed [128, 158]. In bovine tuberculosis, high levels of circulatory IFNγ are detected at the early stage of disease that could inhibit Mycobacterial growth, suggesting that the host most likely mounts an early Th1 response [166–168]. However, in the chronic tuberculosis increased serum IgG1 (a Th2 associated antibody) is detected in the serum [128]. In line with these observations, in mice and humans, IFNγ expression was upregulated during the early phases of tuberculosis, however, at the chronic phase IL-4 expression was enhanced [169–172]. Collectively, these results suggest that *Mycobacterium tuberculosis* can shift an IFNγ (Th1) dominant response towards an IL-4 (Th0 or Th2) dominant response at the later stages of disease. Interestingly, the frequency of antigen-specific Th0 clones was higher in animals showing severe lung pathology

than in animals having less severe lesions [128]. Therefore, the authors speculated that Th0 clones may play an important role in skewing the immune response from Th1 (IFNγ) response towards Th0 or Th2 (IL-4) response during the progression of infection (**Figure 4**) [128]. As in *Mycobacterium tuberculosis* infections, the immune responses to *Mycobacterium paratuberculosis* switches from Th1 response to Th2 response while the disease progresses from subclinical to clinical stage [158]. In *Mycobacterium paratuberculosis* infections, cattle show high levels of IFNγ in the supernatant of cultured PBMCs and high levels of IFNγ mRNA in the intestinal ileal tissues, suggesting an induction of Th1 response against this pathogen [173, 174]. Importantly, cattle clinically infected with *Mycobacterium paratuberculosis* had significantly lower expression of IFNγ in ileal and caecal lymph nodes compared to cattle at sub-clinical stage of infection [175]. This finding supports the notion that the suppression of the Th1 response at the sub-clinical stage of the disease might have contributed to the progression of disease into the clinical stage. Furthermore, increased antigen-specific IgG1 was detected in animals infected with *Mycobacterium paratuberculosis* at the clinical stage, suggesting a Th2 response [176, 177]. Together, these findings suggest that the shift of an early-induced Th1 dominant response towards a Th0 or Th2-dominant response is associated with disease progression in both bovine tuberculosis and bovine paratuberculosis.

During the early phases of Respiratory syncytial virus (RSV) infection in humans and mice, the host launches a Th1/Th2 mixed response (*i.e.,* both IFNγ and IL-4), which then shifts towards a Th2 response (*i.e.,* increased circulatory IL-4) during chronic infection [178–180]. Consistently, cattle infected with Bovine respiratory syncytial virus (BRSV) seem to mount a Th0 response, which turns into a Th2 response during chronic infection [143, 181]. In the past, reports suggested that both IFNγ and IL-4 were detected in the peripheral blood, lymph sample and pulmonary tissues of BRSV infected animals at the early stage, indicating the induction of a Th0 response [144, 181, 182]. Similarly, both IgG1 and IgG2 were detected in the serum, although they peaked at different times during infection [182]. Conversely, IgE and IgG1 levels increased as the infection progressed towards the chronic stage, suggesting a gradual shift from a Th0 towards a Th2 response [143, 181–183]. Collectively, these studies indicate that these pathogens can switch the early-induced Th0 response towards a Th2 response during chronic infection.

The efficacy of Th0 responses in controlling infections caused by bovine intracellular pathogens is unclear. While Th0 responses seem ineffective against some bovine diseases such as tuberculosis, they can be protective against bovine babesiosis and non-cytopathic Bovine viral diarrhea virus (ncp- BVDV) infection [156, 157, 184]. In Babesiosis, both CD8+ T cell responses and humoral responses appear critical to clear infection. For instance, increased numbers of antigenspecific CD8+ T cells were detected in the peripheral blood of vaccinated animals [156]. Similarly, transferring serum from an immune animal containing both IgG1 and IgG2 can clear infection of sick animals [184]. In this regard, *in vitro* experiments have demonstrated that the majority of Babesia-specific clones are Th0, which are able to stimulate B cells to produce both IgG1 and IgG2 [22, 138, 184]. Furthermore, IgG1 and IgG2 antibodies were found effective to prevent invasion of bovine erythrocytes by *Babesia bovis* merozoite *in vitro* [185]. Collectively, these findings suggest that Th0 responses promote both the cytotoxic activity of CD8+ T cells, and neutralizing activities of IgG subtypes [156].

Cattle might launch different immune responses against different biotypes of the same intracellular pathogen [145, 186, 187]. For instance, while Th0 response was induced against the non-cytopathic (ncp) biotype of Bovine viral diarrhea virus (BVDV), Th1 response was induced during infection caused by the cytopathic biotype (cp) [188]. In experiments with T cells isolated from the ncp-BVDV

## *CD4+ T Cell Responses to Pathogens in Cattle DOI: http://dx.doi.org/10.5772/intechopen.100410*


### **Table 2.**

*Characterization of helper T cell responses in diseases induced by bovine intracellular pathogens. Th1/Th2 cytokines were detected in cultured PBMCs and DLNs; IgG subtype was tested in the serum.*

infected cattle, IL-4 protein in the supernatant of CD4+ T cell culture and IFNγ protein in CD8+ T cell culture were detected, suggesting possible induction of Th0 response [157]. More recently, Palomares et al. analyzed cytokine expression in tracheo-bronchial lymph nodes and found that both IFNγ and IL-4 were detected in ncp-BVDV-infected cattle, but IL-12 mRNA was only detected in cp-BVDV-infected cattle [145]. Additionally, while only IgG2 was detected in the serum of cp-BVDV-infected cattle, both IgG1 and IgG2 were detected in ncp-BVDV infected cattle after day 35 of infection [187]. These results collectively reveal that ncp-BVDV induces a Th0 response whereas cp-BVDV induces a Th1 response in infected cattle.

Thus, available literature supports the notion that cattle launch either Th1 or Th0 responses against most infectious diseases caused by intracellular pathogens (**Table 2**). Moreover, although further research is required to confirm these findings, the shift from an early Th1 or Th0 response towards a Th2 response is associated with progression of disease towards chronic condition.

## **4.2 Most extracellular pathogens induce either a Th2 or Th0 response in cattle**

In mice and humans, Th2 responses are typically effective in controlling extracellular pathogens. In this regard, Th2 cytokines can induce processes such as IgG subtype switching and migration of mast and eosinophils to the site of infection that are critical for defending the host against extracellular bacteria and parasites [98]. In cattle, most of extracellular parasites induce either Th2 or Th0 responses [193–195]. However, some pathogens are capable of suppressing Th2 response, which is associated with the establishment of chronic infections [196].

Generally, Th2 responses are effective in controlling gastrointestinal nematodes such as *Cooperia oncophora* [197, 198]. Infected animals had increased level of antigen-specific IgG1 (Th2 associated antibody) in the serum [199]. Consistently, a high titer of pathogen specific IgG1 was associated with a better immune response [200]. Similarly, increased numbers of peripheral eosinophils (a Th2 response feature) was associated with increased expulsion of cooperial larvae [200]. Importantly, cytokine analysis of the intestinal tissue of disease resistant cattle demonstrated high expression level of IL-4 and IL-13 mRNA compared to those susceptible animals [201, 202]. These results offer compelling evidence that Th2 response is critical to control infection caused by some extracellular pathogens such as *Cooperia oncophora*.

Interestingly, some extracellular parasites such as *Dictyocaulus viviparus* (lung worm) are capable of shifting the initial Th2 or Th0 response into an ineffective Th1 response to establish chronic infections [203, 204]. At the early stage, both IL-4 and IFNγ were detected in the lungs and DLNs after day 15 of lung worm infection, indicating an initial Th0 response [205]. However, subsequent research only detected increased IL-4 mRNA for a short period of time in the Broncho-alveolar lavage fluid (BALF) of infected cattle, suggesting a possible Th2 response [206]. In line with this finding, high level of total IgE (antigen-specific plus non-specific) in the serum and BALF was associated with the clearance of lungworm [203]. Furthermore, in the chronically infected animals the detection of Th1 associated antibody (*i.e.,* IgG2) in the serum, was associated with increased lungworm larval excretion [204]. These data indicate that bovine lungworm might shift the earlyinduced Th0 or Th2 response towards a Th1 dominant response to establish chronic infection.

In cattle *Fasciola hepatica* (liver fluke) can modulate the early-induced Th1 or Th0 response into an ineffective Th2 response at the later phases of the disease [207, 208]. Of note, although an initial Th1 response was observed in the peripheral blood, a Th0 response was also observed inside the hepatic lymph node, as indicated by the detection of both IFNγ and IL-4 [209–212]. Collectively, these experiments suggest that cattle might launch either a Th1 or a Th0 response at the early stages of liver fluke infection. However, at later stages, the response is shifted to a Th2 response as indicated by the significantly increased expression of IL-4 mRNA (x6) and significantly reduced expression of IFNγ mRNA (x6) in the hepatic tissue of infected animals, which is consistent with several other reports [140, 213, 214]. In line with these observations, peripheral blood lymphocytes obtained from chronically infected animals failed to induce IFNγ secretion when co-cultured with adult fluke antigen *in vitro* [209]. Importantly, chronically infected cattle typically show high levels of antigen-specific IgG1 in the serum [140]. Altogether, these findings suggest that *Fasciola hepatica* might switch a Th1 or a Th0 dominant response to a Th2 dominant response at the chronic stage of disease.

*Ostertagia ostertagi* (OO), an economically important abomasal nematode, typically induces Th0 response [215]. Bovine OO usually causes chronic infection and requires long-term repetitive exposure (at least 2 years) to develop effective immunity [216]. Both pathogen-specific IgG subtypes (IgG1 and IgG2) were detected in OO-infected cattle, with higher serum IgG1 titer than IgG2 [217]. Similarly, mRNAs of both IL-4 and IFNγ were upregulated in the abomasal lymph nodes of experimentally infected cattle from day 11 to day 28 after infection, suggesting the induction of a Th0 responses [215]. In contrast to this observation, subsequent research demonstrated induction of Th2 response in the abomasal lymph nodes of OO infected cattle [218]. The differences observed between these two experiments might be explained, at least in part, by the differences in time points for cytokine detection and in the number of L3 larvae used for experimental infection. More specifically, while Canals et al. measured cytokine expression from day 11 to day 28 post infection and used 200,000 L3 larvae for experimental infection, Claerebout (2005) measured cytokine expression after 8 weeks post primary infection and only used 25,000 L3 larvae [215, 219]. Recently, Mihi et al. experimentally infected cattle with 200,000 L3 larvae and tested the gene expression of Th1/Th2 cytokines at different time points; interestingly, the authors observed a positive association between upregulation of both IFNγ and IL-4 (in mucosa) with migration of adult (L5) worms out of gastric gland towards abomasal mucosa [146]. These observations suggest that *Ostertagia ostertagi* may modulate the bovine immune response by inducing a Th0 response, which is ineffective in controlling OO and leads to the establishment of chronic infections.

### *CD4+ T Cell Responses to Pathogens in Cattle DOI: http://dx.doi.org/10.5772/intechopen.100410*

