Blood monocytes

Blood monocytes can be divided into two subsets: those expressing CD14 (a receptor for lipopolysaccharide, a bacterial cell wall component) and those expressing CD14 and CD16 (a receptor for IgG antibodies). In vitro, both subsets have the potential to differentiate into myeloid dendritic cells (after culture with IL-4 and granulocyte-monocyte colony simulating factor, GM-CSF) and into macrophages, which may exist in specialized forms (e.g. alveolar and gut macrophages and osteoclasts).

Tissue macrophages

A key role of tissue macrophages is the maintenance of tissue homeostasis, through clearance of cellular debris, especially following infection or inflammation. They are responsive to a range of pro-inflammatory stimuli, using their pattern recognition receptors (PRR) to recognize pathogen-associated molecular patterns (PAMPs). Once activated, they engulf and kill microorganisms, especially bacteria and fungi. In doing so they release a range of pro-inflammatory cytokines and have the capacity to present fragments of the microorganisms to T lymphocytes (see below) in a process called antigen presentation. Recent evidence suggests that evolutionarily conserved molecular patterns in mitochondria (organelles that originally derived from bacteria) can also activate monocytes. These damage-associated molecular patterns (DAMPs) could play a major role in the systemic inflammatory response that follows extensive tissue damage (e.g. following ischaemic injury).

It has been observed that some PAMPs induce the cytoplasmic assembly of large oligomeric structures of PRRs termed inflammasomes. There are numerous examples: members of the Nod-like receptor (NLR) family can be activated by stimuli such as viruses, bacterial toxins, and interestingly, crystallized endogenous molecules, including urate. Inflammasomes have potent effects in activating caspases, leading to processing and secretion of pro-inflammatory cytokines such as IL-1β and IL-18.

Macrophages have pro-inflammatory and microbicidal capabilities similar to those of neutrophils. Under activation conditions, antigen presentation (see p. 57, 58) is enhanced and a range of cytokines secreted, notably TNF-α, IL-1 and IFN-γ. These are necessary for the removal of certain pathogens that live within mononuclear phagocytes (e.g. mycobacteria). Macrophages and related cells may also undergo a process termed autophagy (p. 32, Ch. 2). This self-cannibalization is a critical property of many cell types under starvation conditions, but is used by the immune system to destroy intracellular pathogens such as Mycobacterium tuberculosis, which otherwise persist within cells and block normal antibacterial processes. Autophagy is also a means of enhancing antigen presentation pathways.

Tissue macrophages involved in chronic inflammatory foci may undergo terminal differentiation into multinucleated giant cells, typically found at the site of the granulomata characteristic of tuberculosis and sarcoidosis (see p. 845).

Types of dendritic cell

The major types are the myeloid DC (mDC), the plasmacytoid DC (pDC) and a variety of specialized DCs found in tissues that resemble mDCs (e.g. the Langerhans cell in the skin, see Fig. 24.1). DCs have several distinctive cell surface molecules, some of which have pathogen-sensing activity (e.g. the antigen uptake receptor DEC205 on mDCs) whilst others are involved in interaction with T lymphocytes (Table 3.7). Immature mDCs and pDCs are present in the blood, but at very low levels (<0.5% of lymphocyte/monocyte cells).

Table 3.7 Molecules on dendritic cells

Pathogen sensing is a key component of the function of immature DCs, as well as monocytes/macrophages, and is achieved through expression of a limited array of specialized PRR molecules capable of binding to structures common to pathogens, aided by long cell dendrites and pinocytosis (constant ingestion of soluble material).

PRRs include:

Table 3.8 Toll-like receptors (TLR)

The key principle at play here is that the immune system has devised a means of identifying most types of invading microorganisms by using a limited number of PRRs recognizing common molecular patterns, or PAMPs. This recognition event has been termed a ‘danger signal’: it alerts the immune system to the presence of a pathogen. Sensing danger is a key role of the DC and a key first step towards activation of the adaptive immune system.

DCs and T cell activation

In a sequence of events that spans 1–2 days, immature DCs are activated by PAMPs or DAMPs in the tissues binding to a PRR on DCs. The immature pDC is a small rounded cell that develops dendrites upon activation and secretes enormous quantities of IFN-α, a potent antiviral and pro-inflammatory cytokine. On activation, the DC migrates to the local lymph node with the engulfed pathogen. During migration the DC matures, changing its shape, gene and molecular profile and function within a matter of hours to take on a mature form, with altered functions (Table 3.9, Fig. 3.5), in particular upregulating machinery required to activate T lymphocytes. Once in the lymph node, the mature DC interacts with naive T lymphocytes (antigen presentation), resulting in two key outcomes:

Table 3.9 Myeloid dendritic cell (DC) maturation

Figure 3.5 (a) Immature dendritic cells (DCs) in the tissues are activated by pathogens through PAMP–PRR interaction. (b) Multiple rapid changes in gene expression lead to migration to the lymph node as the DC takes on the mature phenotype. During migration there is synthesis of the machinery required for activation of T lymphocytes, shown here in response to signals 1–3.

The mature DC provides three major signals to naive T cells Fig. 3.5):

HLA classes

Classical class I HLA genes (also termed Ia), are designated HLA-A, HLA-B and HLA-C. Each encodes a class I α chain, which combines with a β chain to form the class I HLA molecule (Fig. 3.6). While there are several types of α chains, there is only one type of β chain, β₂ microglobulin. The HLA class I molecule has the role of presenting short (8–10 amino acids) antigenic peptides to the T cell receptor on the subset of T lymphocytes that bear the co-receptor CD8. As an example of HLA polymorphism, there are nearly 200 allelic forms at the A gene locus. Class I HLA molecules are expressed on all nucleated cells.

Non-classical HLA class I genes are less polymorphic, have a more restricted expression on specialized cell types, and present a restricted type of peptide or none at all. These are the HLA-E, F and G (Ib genes) and MHC class I-related (MIC, or class Ic) genes, A and B. The products of these genes are predominantly found on epithelial cells, signal cellular stress and interact with lymphoid cells, especially natural killer cells (see p. 61, 62).

The class II genes have three major subregions, DP, DQ and DR. In these subregions are genes encoding A and B genes that combine to form dimeric αβ molecules that present short (12–15 amino acid) peptides to T lymphocytes that bear the CD4 co-receptor. Class II HLA genes (apart from DRA) are highly polymorphic. Other genes in this region encode proteins with key roles in antigen presentation (e.g. TAP, HLA-DM, HLA-DO, proteasome subunits; see below). Class II HLA genes are expressed on a restricted cohort of cells that go by the general term of antigen presenting cells (APCs; DCs, monocyte/macrophages, B lymphocytes).

HLA class III genes encode proteins that can regulate/modify immune responses, e.g. tumour necrosis factor (TNF), heat shock protein (HSP) and complement protein (C2, C4).

HLA genotypes and the range of their protein products

HLA genotype is denoted first by the letters that designate the locus (e.g. HLA-A, HLA-DR, HLA-DQ). For class I alleles, this is followed by an asterisk and then a 2- to 4-digit number defining the allelic variant at that locus, often called the HLA type (e.g. HLA-A*02 is the 02 variant of the HLA-A gene). The class II nomenclature is the same, except that both A and B genes are named (although HLA-DR molecules only require the name of the B gene, because the A gene is the same in all of us).

Some general principles apply to the HLA genes and their protein products:

Overall, then, each human can bind a range of peptide epitopes from pathogens to enhance individual protection, and similarly the population has an even greater range of protection, to ensure population survival.