Introduction

Living organisms are continuously exposed to infectious agents such as viruses, bacteria and other pathogens. Without an immune system, animals would soon die from disease caused by these infectious agents. The immune system includes all the mechanisms available to protect the body against disease caused by these agents and neoplastic cells. The immune system is extremely adaptable and can generate a great diversity of cells and molecules able to recognize and eliminate a large variety of potential pathogens. From a functional point of view, two sub-divisions can be considered: the innate (or non-specific) immunity that confers basal resistance to illness, immediately functional at birth and the first-line defense; and the adaptive (or acquired) immunity, which generates specific reactions, with immunological memory, against each potential biological invader.

Innate immunity includes anatomical barriers (skin, mucous membranes, bronchial epithelial cells) and physiological reaction (fever, pH variation), as well as humoral and chemical transmitters such as interferons, lysozyme, inflammatory proteins, cytokines, complement and others. Lastly, it includes specialized cells such as phagocytes (neutrophils, monocytes and macrophages), killer cells, called natural killers or NK, and lymphokine-activated killers or LAK. The innate inflammatory response is orchestrated by cytokines. These small signaling proteins secreted by numerous – immune and other – cells ensure intercellular communication. They bind to specific receptors and induce a response in the target cells, modulating cell function by modifying its gene expression. Cytokines can have pro-inflammatory effects (e.g. IL-6, IL-8, IL-12, TNF-α) or anti-inflammatory effects (IL-10). Others have antiviral effects (INF type 1) or play a regulating role of the adaptive immunity (IL-4, IL-7, IL-23).

Acquired immunity is mediated by humoral and cellular components. Specialized cells are effector T cells such as cytotoxic T lymphocytes that have both antigen specificity and immunological memory. T helper cells regulate the adaptative response by producing cytokines. Lymphocytes B and T secrete a broad panel of substances including compounds such as antibodies and cytokines.

Antigen-presenting cells (APCs) represent the link between both immune systems. They are efficient at internalizing antigens, either by phagocytosis or endocytosis: they then display fragments of foreign antigen bound with a class II major histocompatibility complex (MHC) located on their membrane. The T cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen-presenting cell. An additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the T cell. Dendritic cells have the broadest range of antigen presentation, and are probably the most important APC.

Exercise is recognized as one of the most potent physiological stresses that a healthy organism might experience. The question of knowing if this stress is compatible with health and whether it influences immunity is still poorly elucidated in humans and horses.

The relationship between exercise, immune function and infectious or inflammatory diseases remains poorly resolved.1–4 The difficulty of assessing this relationship is due to numerous factors, among which the complexity of the immune function, the variability of the exercise considered (which varies by equestrian discipline, intensity and duration), the population studied (sedentary, athletic, young, mature, trained or untrained), what function or what cells have been analyzed to assess the immunity, the timing of sampling used (during exercise, just after, or 1, 24 or 48 hours after); and the difficulty of dissociating the effect of the effort from other effects (psychological stresses, environment). Last but not least, it is difficult to determine the genuine clinical significance of the changes observed in some parameters of the immunity.

Assessment of immunity

The relationship beteween immunity and exercise is evaluated through either epidemiological studies or experimental biocellular or biomolecular studies.

Epidemiological studies help to assess the effect of physical activity on a given population but do not bring any mechanistic information.

Experimental studies focus on one isolated component of the immune function and help to understand some mechanisms. Nevertheless, it is often difficult to assess the clinical relevance of these findings as they might not reflect the overall immune status.

The general principle of the experimental studies related to immunity is the sampling of biological fluids, leukocytes, or both, to evaluate:

Recent discoveries in molecular biology have opened new prospects for studying the non-specific response of the first-line defender cells when microorganisms invade the organism. Among these findings, are pattern recognition receptors (PRR) such as Toll-like receptors (TLRs) and NOD-like receptors (NLRs) that seem to play a key role in the defense against microbes.5,6 These receptors are activated after recognizing specific pathogen-associated molecular pattern (PAMP) or damage-associated molecular pattern (DAMP). When these receptors bind and recognize their specific ligand, they induce a cascade of events leading to the activation of transcription factors, resulting in production of chemokine/cytokine, upregulation of cell adhesion molecules, phagocytic cellular infiltration, and finally clearance of the microorganisms.

The study of TLRs activation by specific ligands is being investigated in equids.7–10 For example, TLR4 is located in the cellular membrane and is involved in the recognition of several bacterial signals. The TLR4 cascade is activated by the classical addition of bacterial endotoxins (lipopolysaccharide or LPS) on cultured peripheral blood mononucleated cells (PBMC) or pulmonary alveolar macrophages (PAM) and results in the production of pro-inflammatory cytokines such IL-1α, IL-6 and TNF-α. These tests, overall, evaluate the response of the phagocytic cells in front of a bacterial attack.

In young racehorses in training, respiratory problems are frequently related to viral diseases (EHV4, influenza) which induces acute respiratory disease with temporary incapacity to work, and that can ‘prepare the ground’ for secondary infectious and/or inflammatory complications.¹¹ It is therefore of paramount importance to examine the capacity of the immune system to protect against disease caused by these viruses. Polyinosinic-polycytidylic acid (poly I : C) has a structure similar to the double strand of RNA contained in some viruses and, consequently, is a tool used in research to evaluate the immune response against viral attack. It is a stimulant of the Toll-like receptor type 3 (TLR3), an intracellular receptor included in the endosome membrane and which is involved in the recognition of viral DNA/RNA. It is expressed in lymphocytes T cells, dendritic cells and phagocytes. The recognition of its ligand will mainly result in the production of INFβ. Experimental induction of the TLR3 pathway can be obtained by adding poly I : C (which has a structure similar to the viral RNA) on cultured PBMC, or PAM or bronchial epithelial cells (Fig. 45.1).

Current research is aimed at studying the response of PBMC, PAM and bronchial epithelial cells to specific stimulation of TLR with synthetic ligands. The cells are put in a culture medium and stimulated by LPS (specific of TLR4), poly I : C (specific of TLR3), and other specific ligands for the different TLRs (Table 45.1). The cytokines in the supernatant are then analyzed (when ELISA homologous for the horses is available) and the white cells are recovered for extraction of RNA and transcriptomic study (when ELISA are not available). These methods have shown a probable decrease of resistance to virus 24 hours after an exercise to fatigue in Thoroughbred horses.¹⁰

Epidemiologic data

Epidemiological studies carried out in man have led to the following observations:

• Moderate regular physical activity confers a protection against chronic inflammatory diseases (cardiac, type 2 diabetes, chronic obstructive pulmonary disease) and reinforces the resistance of the subjects to the infectious viral and bacterial diseases. This is especially true in the elderly.3,4

• Intense physical activity, and particularly a competition, induces an increased (+150%) susceptibility to upper airway respiratory diseases, just after the exercise. Moreover, intense exercise seems to reactivate latent infections.3,12,13 (see Fig. 45.2).

• The susceptibility to infections of the upper airway is greater in ‘overtrained’ subjects.¹²

• There is a much higher prevalence of asthma and bronchitis in human athletes, especially in marathon runners, than in the sedentary population. Air quality and characteristics (chemical, biological and physical) as well as high level of ventilation are thought to be favoring factors.⁴

In horses, epidemiological studies have also shown an increased susceptibility to infectious disease in young racehorses, despite appropriate vaccination.¹¹

Another characteristics of racehorses regularly trained and involved in competition is the fact that they frequently suffer from chronic inflammation of the airway, a syndrome commonly known as inflammatory airway disease (IAD).¹⁴ The primum novens and the mechanisms of occurrence of IAD remain poorly defined to date. Most probably, the problem is related to an inappropriate response of the innate immune system. Dysregulation of immunity in IAD could be due either to an uncontrolled chronic inflammation or, on the contrary, a subdued inflammatory response, favoring the occurrence of secondary microbial and fungal infections. The fact that some scientists propose to treat IAD by anti-inflammatory drug administration while others recommend to control it by giving stimulants of immunity gives an idea of the extent of the lack of knowledge on this syndrome (Fig. 45.3).