Immunity, Inflammation and Infection

Innate Immunity

The innate system produces a semi-specific response to newly encountered organisms. It is also essential to triggering adaptive responses via signalling cytokines. Macrophages and dendritic cells patrol the tissues for foreign proteins likely to indicate infection. Invaders bearing foreign proteins are engulfed and destroyed by antimicrobial molecules and the complement system is activated. Once engaged, the TLRs on the cell surface prompt the cells to unleash particular suites of cytokines, which then recruit additional macrophages, dendritic cells and other immune cells to contain and destroy the infecting organisms. Dendritic cells containing engulfed protein then transit to lymph nodes, where they present fragments of the pathogen’s protein to an array of T cells and release more cytokines. Lipopolysaccharide (LPS) produced by gram-negative bacteria is a particularly powerful immune stimulator. It prompts inflammatory cells to release tumour necrosis factor alpha (TNF-alpha), interferon and interleukin-1 (IL1) . These cytokines are probably the most important in controlling the inflammatory response, and also, if unchecked, in causing autoimmune disorders, for example, rheumatoid arthritis.

At least 10 human varieties of TLRs are known. They act in pairs and each pair binds to a different class of protein characteristic of a type or group of organisms, for example, gram-negative bacteria, single-stranded deoxyribonucleic acid (DNA) viruses or flagellin. The released cytokines generate the typical symptoms of infection—fever and flu-like symptoms.

Overactivity of this innate system can lead to potentially fatal sepsis . TLRs may also be implicated in autoimmunity by responding inappropriately, for example to damaged cells. A range of drugs that activate particular TLRs are in advanced stages of testing, for example, as vaccine adjuvants or antiviral agents. Inhibitors are also under development for treating sepsis, inflammatory bowel disease and autoimmune diseases, so far with limited success.

Adaptive Immunity

Macrophages and other antigen-presenting cells, having ‘processed’ a pathogen, display fragments on their surface. This ultimately activates B and T cells that recognise that fragment to proliferate, and thereby initiate a powerful and highly focused immune response. Activated B cells secrete antibody molecules that bind to unique antigen components and destroy the target or else mark it for destruction. T cells recognise antigens displayed on cells. Some activate more B and T cells whilst others directly attack infected cells. Following the initial infection, enough memory T and B cells remain to deal effectively with the organism, should it return. This can occur so quickly that inflammation may not occur.

The Microbiome and Surgical Disease

The gut microbiome is now believed to play a critical role in the aetiology of some chronic disease states. The microbiome implies a network effect of interacting organisms rather than a direct relationship between single organisms and disease states as in Koch postulates. The gut microbiome is highly individual and varies through age, reaching its adult structure by the age of 3 years. Abnormalities have been incriminated in a range of conditions including cancer, obesity, diabetes, autoimmune diseases and neuropsychiatric disorders. Widespread use of antibiotics and proton-pump inhibitors changes the structure and function of the gut microbiome, thereby influencing health in adult life, although the extent and mechanisms of the changes are as yet ill understood.

For surgeons, the gut microbiome appears involved in the aetiology of colonic diseases such as diverticulosis(itis), inflammatory bowel syndrome and cancer. Sporadic colorectal cancer (CRC) is the third most common cause of cancer-related death worldwide and its incidence is increasing. There is strong epidemiological evidence that diet is a major risk factor (high in red meat and fat, and low in fibre), but data now suggest the colonic microbiota and its metabonome is an important driver of CRC risk. One mechanism is through its modulation of dietary fibre, resulting in upregulation of butyrate metabolism and reduction in secondary bile acid metabolism. Another idea is that certain microbiome members may produce prooncogenic carcinogens and promote a mucosal immune response and colonic epithelial cell changes that initiate colorectal carcinogenesis.

Supersystem and Surgery

The gut microbiome is a complex supersystem, and all surgically related interventions that disturb it have implications for the host. For example, the microbiome may play a critical role in anastomotic healing, postoperative ileus, surgical nutritional status and the systemic inflammatory response syndrome (SIRS).

The gut microbiome probably also plays a fundamental role in the host response to chemotherapeutic drugs for tumour types anywhere in the body, by facilitating drug efficacy, compromising anticancer effects and mediating toxicity. The modern surgeon needs to be aware of the potential impact of bowel preparation, antibiotics and surgical resection on the oncological function of the gut.

Acute Inflammation

Introduction

Acute inflammation is the principal mechanism by which living tissues respond to injury. The purpose is to neutralise the injurious agent, to remove damaged or necrotic tissue and to restore the tissue to useful function. The central feature is formation of an inflammatory exudate with three principal components: serum , leucocytes (predominantly neutrophils) and fibrinogen .

Formation of inflammatory exudate involves local vascular changes collectively responsible for the four ‘ cardinal signs of Celsus ’—rubor (redness), tumour (swelling), calor (heat) and dolor (pain)—as well as loss of function. These vascular phenomena are described in Fig. 3.1 . The outcomes of acute inflammation are summarised in Fig. 3.2 .

Resolution

If tissue damage is minimal and there is no actual tissue necrosis, the acute inflammatory response eventually settles and tissues return virtually to normal without evidence of scarring. A good example is the resolution of mild sunburn.

Abscess Formation ( Fig. 3.3 )

An abscess is a collection of pus (dead and dying neutrophils plus proteinaceous exudate) walled off by a zone of acute inflammation. Acute abscess formation particularly occurs in response to certain pyogenic microorganisms that attract neutrophils but are resistant to phagocytosis and lysosomal destruction. Abscesses also form in response to localised tissue necrosis and to some organic foreign bodies (e.g., wood splinters, linen suture material). The main pyogenic organisms of surgical importance are Staphylococcus aureus , some streptococci (particularly Streptococcus pyogenes ), Escherichia coli and related gram-negative bacilli (‘coliforms’), and Bacteroides species (spp.).

Without treatment, abscesses eventually tend to ‘ point ’ to a nearby epithelial surface (e.g., skin, gut, bronchus), and then discharge their contents. If the injurious agent is thereby eliminated, spontaneous drainage leads to healing. If an abscess is remote from a surface (e.g., deep in the breast), it progressively enlarges causing much tissue destruction. Sometimes local defence mechanisms are overwhelmed, leading to runaway local infection ( cellulitis ) and sometimes sepsis.

Even with small, well-localised abscesses, showers of bacteria may enter the general circulation ( bacteraemia ) but are mopped up by hepatic and splenic phagocytic cells before they can proliferate. This is responsible for the swinging pyrexia characteristic of an abscess. The abscess site may not be clinically evident if deep-seated (e.g., subphrenic or pelvic abscess) and the patient may be otherwise well. In the presence of an abscess, circulating neutrophils rise dramatically as they are released from the bone marrow; thus a marked neutrophil leucocytosis (i.e., white blood cell [WBC] greater than 15×10 ⁹ /L with more than 80% neutrophils) usually indicates a pyogenic infection. Severe infection causing excessive cytokine responses spilling over into the systemic circulation causes sepsis and rapid clinical deterioration (see Chapters 2 and 3).

The Chronic State

(see Chronic inflammation , p. 34)

The essence of managing any abscess is to establish complete drainage , usually by incision or aspiration. Any residual necrotic or foreign material needs to be eliminated by curettage or excision. If drainage of an abscess does not eliminate the injurious agent, the neutrophil response persists and pus continues to be formed, resulting in a chronic abscess .

Antibiotics and Abscesses

If appropriate antibiotics are given early enough, organisms can be eliminated before abscess formation. In surgical operations with a particular risk of infection, therefore prophylactic antibiotics dramatically reduce abscess formation and other infective complications. However, once an abscess has fully formed, antibiotics seldom effect a cure because pus and necrotic material remain and the drug cannot gain access to the bacteria within. Nevertheless, antibiotics may halt expansion or even sterilise the pus; the residual sterile abscess is known as an antibioma .

Organisation and Repair

The most common sequel to acute inflammation is organisation , in which dead tissue is removed by phagocytosis and the defect filled by vascular connective tissue known as granulation tissue . This tissue is gradually ‘repaired’ to form a fibrous scar . Sometimes the original tissue regenerates, that is, rebuilds its specialised cells and structure.

Wound Healing

Healing by Primary Intention

The simplest example of organisation and repair is healing of an uncomplicated skin incision ( Fig. 3.4 ). There is no necrotic tissue and the wound margins are brought into apposition with sutures. An acute inflammatory response develops in the vicinity of the incision, and by the third day, granulation tissue bridges the dermal defect. In the meantime, epithelium proliferating rapidly from the wound edges restores the epidermis. Fibroblasts invade the granulation tissue, laying down collagen so the repair is strong enough for suture removal after 5 to 10 days. The scar is still red but blood vessels gradually regress and it becomes a pale linear scar within a few months. This is known as healing by primary intention .

Healing by Secondary Intention

If tissue loss prevents the wound edges from coming together, healing has to bridge over the defect, which is initially filled with blood clot. This later becomes infiltrated by vascular granulation tissue from the healthy wound base. Inflammatory exudate solidifies, forming a protective scab. Fibroblasts invade and lay down collagen in the extracellular spaces; after about a week, some fibroblasts differentiate into myofibroblasts and contraction of their myofibrils eventually shrinks the wound defect by 40% to 80%, beginning about 2 weeks after the injury. Over the weeks and months, blood vessels regress and more collagen is formed, leaving a relatively avascular scar; gradual contraction of the mature collagen (cicatrisation), combined with wound contraction, ensures the final scar is much smaller than the original defect. The epidermal defect is gradually bridged by epithelial proliferation from the wound margins. Epithelial cells slide over each other beneath the edges of the scab on the granulation tissue surface and the scab is eventually shed. This whole process is known as healing by secondary intention (see Fig. 3.4 ).

Factors Impairing Wound Healing

The rate and success of wound healing may be impaired by a variety of local, regional and systemic factors ( Fig. 3.5 ).

Chronic Inflammation

Sometimes an injurious agent persists over a long period causing continuing tissue destruction. The body attempts to deal with the original and the continuing damage by acute inflammation, organisation and repair, all at the same time. The damaged area may display several pathological processes at once, that is, tissue necrosis, an inflammatory response, granulation tissue formation and fibrous scarring. This is known as chronic inflammation and is characterised histologically by a predominance of macrophages (sometimes forming giant cells), responsible for phagocytosis of necrotic debris. Lymphocytes and plasma cells are also present, indicating immunological involvement in chronic inflammation.

Chronic inflammation represents a tenuous balance between a persistent injurious agent and the body’s reparative responses. Healing only occurs if the injurious agent is removed and then proceeds in the usual manner but often with much more scarring.

A range of agents can lead to chronic inflammation. The clinical patterns can be grouped into three categories:

• Chronic abscesses

• Chronic ulcers

• Specific granulomatous infections and inflammations

General Principles

It is important to distinguish between colonisation, infection and sepsis:

• Colonisation is when bacteria are present in or on a host but do not cause an immune response or signs of disease

• Infection occurs when microorganisms provoke a sustained immune response and signs of disease, for example, when normal commensal bacteria in the colon such as Escherichia coli contaminate the peritoneal cavity

• Sepsis (systemic sepsis) is the result of an excessive and inappropriate production of cytokines in response to severe infection or tissue necrosis (e.g., a gangrenous limb) that causes organ dysfunction and progressive organ failure

Clinically significant infection arises when the size of an inoculum or the virulence of a microorganism is sufficient to overcome the innate and adaptive immune responses and lead to symptoms. The virulence of an organism depends on its qualities of adherence and invasiveness and its ability to produce toxins. Tissue invasion of microorganisms may be enhanced by their secretion of enzymes (e.g., hyaluronidase and streptokinase), by mechanisms to avoid phagocytosis (e.g., encapsulation or spore formation), by inherent resistance to lysosomal destruction or by their ability to kill phagocytes. Toxins may be secreted by the organism ( exotoxins ) or released upon the death of the organism ( endotoxins ). In either case the toxin may produce local tissue damage (e.g., gas gangrene), cause distant toxic effects (e.g., tetanus), or activate cytokine systems to cause sepsis (e.g., disseminated intravascular coagulopathy).

Infections may be community-acquired (e.g., pneumococcal lobar pneumonia in a fit young adult) or hospital-acquired . The latter are also known as nosocomial infections and are defined as infections not present or incubating at the time of hospital admission. A third category is healthcare–associated infection (HCAI) in patients making frequent contact with healthcare institutions or in long-term care. Nosocomial and HCAI may be acquired by cross-infection from infected patients, from contaminated furnishings, or from ‘ carriers ’ among staff by inhalation, ingestion or through contamination of medical equipment and devices, such as intravenous cannulas or urinary catheters. These infections are often caused by antibiotic-resistant bacteria, such as methicillin-resistant Staphylococcus aureus ( MRSA ). Risk of such infections can be drastically reduced by the simple measure of everyone in contact with patients washing their hands with soap and water or using alcohol-based gel between every patient contact . Patients or carriers of multiresistant organisms should be isolated when in hospital. Patients having operations where infection carries very high risk should ideally be treated in areas separated from sick patients, especially emergency admissions from long-term care institutions. Particular risk is associated with eye surgery, joint replacements and prosthetic vascular grafts.

Postoperative patients are at particular risk of nosocomial infections (e.g., pneumonias, urinary tract infections) as host defences are impaired by the surgical assault, and physiological protective mechanisms are disrupted allowing infection to gain ascendancy. For example, neutropenia predisposes to infection, and smokers are more liable to develop bronchopneumonia following general anaesthesia. The surgical patient’s general resistance may be further impaired by malnutrition, malignancy, rheumatoid disease, corticosteroids or other immunosuppressive drugs.

Case History

In postsurgical (‘surgical site’) infections, organisms enter the tissues via an abnormal breach of epithelium. This may be surface damage (such as a surgical or traumatic wound or an injection) or result from a perforated viscus. The infecting organisms are often part of the patient’s normal skin, bowel or respiratory tract flora or are normally present in the external environment. For example, Staphylococcus epidermidis is commonly present on skin but causes serious chronic infection of implanted arterial grafts.

Use of Microbiological Tests in Managing Surgical Infections

Surgical infection should be diagnosed clinically and the laboratory used to define its nature and guide antibiotic therapy. Inexperienced junior staff often take swabs which grow organisms in the laboratory, without realising this may be from colonisation and not a clinically important infection. The clinical picture should always determine decisions to treat, although organisms such as Strep. pyogenes may require treatment to prevent cross-infection even if the lesion is mild.

Results of specimens from contaminated sites must be interpreted with caution. Superficial slough or discharge often contains only colonising organisms. For example, in Staph. aureus osteomyelitis, the sinus opening may be colonised by Proteus spp. or Pseudomonas spp. The infecting organism may not be grown unless the wound is cleaned with saline and then swabbed deeply. If possible, a syringe of pus or excised infected tissue should be sent for culture. Samples should ideally be taken before antibiotics are given.

For best results, microbiological specimens should be transported to the laboratory within 2 hours or kept at 4°C.

Microbiological testing for TB and nontuberculous mycobacteria (NTM) requires specific techniques which include prolonged culture. As described earlier, culture and subsequent sensitivity testing is increasingly important in the era of increasing resistance. Polymerase chain reaction (PCR) can be used but the sensitivity and specificity are still not 100%. New technologies are becoming a reality: MALDI-ToF (matrix assisted laser desorption/ionisation—time of flight) enables same-day microbe identification, and molecular techniques, such as 16S PCR allow identification of microbes from culture-negative samples. In general, the microbiology laboratory works most effectively when all relevant clinical information is provided.