**2. CD4+ T cells regulate adaptive immunity**

Naive CD4+ T cells exit the thymus and search for pathogen-derived antigens presented by APCs in secondary lymphoid tissues (*e.g.,* lymph nodes and the spleen). During infections, pathogens break through barriers (Physical, chemical etc) of the host to establish infection in the local tissues [33]. As a result, the immune system in the host initiates an inflammatory response through recruitment of immune cells such as neutrophils to the site of infection, which secretes inflammatory cytokines and chemokines [34, 35]. These chemokines provide signals for further recruitment of APCs to the site of infection. APCs constantly search for invading pathogens through recognizing pathogen associated molecular patterns (PAMPS) on pathogens by their pattern recognition receptors (PRRs) [36]. For example, Toll-like receptor-4 (TLR-4) on APCs can recognize the lipopolysaccharide (LPS) present on the cell membranes of gram-negative bacteria [37]. After recognition, APCs engulf the pathogen, break it down into small peptides, and finally present the peptides to CD4+ T cells in the secondary lymphoid tissue. Recognition of this peptide–MHC-II complex by the TCRs on the naïve CD4+ T cells provides the 1st activation signal, as shown in **Figure 1** [41]. At the same time, co-stimulatory molecules on the CD4+ T cell surface (*e.g.,* CD28) recognize their corresponding ligands on the APC surface (*e.g.,* CD80 or CD86), which provides the 2nd activation signal [42]. The final and 3rd signal, which occurs simultaneously with antigen stimulation and co-stimulation, is provided by cytokines such as Interleukin-12 (IL-12) or Interleukin-4 (IL-4) that not only enhance the activation process, but also drive CD4+ T cell differentiation into a specific subtype (*e.g.,* Th1 or Th2) [2, 43]. Therefore, APCs can provide all 3 signals to naïve CD4+ T cells, which facilitates their activation and differentiation (**Figure 1**). Pathogens can regulate host helper T cell response through targeting any of the three signals directly or indirectly, which will be discussed in Section 5. Recently, we have reported that bovine CD4+ T cells respond to three signals in a way similar to that in humans and mice [44]. Furthermore, IL-12 and neutrophils can work on bovine CD4+ T cells synergistically to enhance their production of IFNγ [44].

### **Figure 1.**

*Three-signal model for CD4 + T cell activation: The 1st signal is provided when TCRs recognize the peptide– MHC-II complex presented by APC; the 2nd signal is initiated when CD28 on CD4+ T cells interacts with CD80/86 on APCs, and the 3rd signal is triggered by cytokines released from the APCs and other cells. CD28/ CD80/CD86 interaction is used as an example. This figure was adapted from previous reviews [38–40].*
