Rachel Allen

Defying death after DNA damage.

DNA damage frequently triggers death by apoptosis. The irreversible decision to die can be facilitated or forestalled through integration of a wide variety of stimuli from within and around the cell. Here we address some fundamental questions that arise from this model. Why should DNA damage initiate apoptosis in the first place? In damaged cells, what are the alternatives to death and why should they be selected in some circumstances but not others? What signals register DNA damage and how do they impinge on the effector pathways of apoptosis? Is there a suborganellar apoptosome complex effecting the integration of death signals within the nucleus, just as there is in the cytoplasm? And what are the consequences of failure to initiate apoptosis in response to DNA damage?

Evolution and transmission of stable CTL escape mutations in HIV infection

Increasing evidence indicates that potent anti-HIV-1 activity is mediated by cytotoxic T lymphocytes (CTLs); however, the effects of this immune pressure on viral transmission and evolution have not been determined. Here we investigate mother–child transmission in the setting of human leukocyte antigen (HLA)-B27 expression, selected for analysis because it is associated with prolonged immune containment in adult infection. In adults, mutations in a dominant and highly conserved B27-restricted Gag CTL epitope lead to loss of recognition and disease progression. In mothers expressing HLA-B27 who transmit HIV-1 perinatally, we document transmission of viruses encoding CTL escape variants in this dominant Gag epitope that no longer bind to B27. Their infected infants target an otherwise subdominant B27-restricted epitope and fail to contain HIV replication. These CTL escape variants remain stable without reversion in the absence of the evolutionary pressure that originally selected the mutation. These data suggest that CTL escape mutations in epitopes associated with suppression of viraemia will accumulate as the epidemic progresses, and therefore have important implications for vaccine design.

Cutting Edge: Leukocyte Receptor Complex-Encoded Immunomodulatory Receptors Show Differing Specificity for Alternative HLA-B27 Structures

We studied recognition of the disease-associated HLA-B27 allele by immunomodulatory receptors encoded within the leukocyte receptor complex. HLA class I are ligands for members of the killer Ig receptor (KIR) and Ig-like transcript (ILT)/LIR/LILR families (the new LILR nomenclature is described at www. gene.ucl.ac.uk/nomenclature/genefamily/lilr.html). Members of these families bound HLA-B27 in both classical and β2 microglobulin-independent forms. Classical complexes bound ILT2, ILT4, and LIR6 transfectants but not ILT1, ILT3, or ILT5. A free H chain form of HLA-B27 bound ILT4 and LIR6. Both forms of HLA-B27 bound KIR3DL1 transfectants. HLA-B27 free H chain bound CD14+ cells in PBL from healthy controls, consistent with ILT4 expression on monocytes. Alternative recognition of different forms of HLA-B27 by KIR or ILT could influence their immunomodulatory function and may imply a role in inflammatory disease.

Primitive Toll signalling: bugs, flies, worms and man

Abstract A current area of interplay between immunologists, geneticists and developmental biologists concerns how Toll receptors assumed their dual roles in pathogen recognition and insect embryo patterning. The development of mechanisms that recognize and control infectious pathogens has been essential for the survival of metazoan organisms. Here, Padraic Fallon and colleagues consider the insights that might be gained from using nematodes to study immune signalling pathways.

Killer Cell Ig-Like Receptor and Leukocyte Ig-Like Receptor Transgenic Mice Exhibit Tissue- and Cell-Specific Transgene Expression

To generate an experimental model for exploring the function, expression pattern, and developmental regulation of human Ig-like activating and inhibitory receptors, we have generated transgenic mice using two human genomic clones: 52N12 (a 150-Kb clone encompassing the leukocyte Ig-like receptor (LILR)B1 (ILT2), LILRB4 (ILT3), and LILRA1 (LIR6) genes) and 1060P11 (a 160-Kb clone that contains ten killer cell Ig-like receptor (KIR) genes). Both the KIR and LILR families are encoded within the leukocyte receptor complex, and are involved in immune modulation. We have also produced a novel mAb to LILRA1 to facilitate expression studies. The LILR transgenes were expressed in a similar, but not identical, pattern to that observed in humans: LILRB1 was expressed in B cells, most NK cells, and a small number of T cells; LILRB4 was expressed in a B cell subset; and LILRA1 was found on a ring of cells surrounding B cell areas on spleen sections, consistent with other data showing monocyte/macrophage expression. KIR transgenic mice showed KIR2DL2 expression on a subset of NK cells and T cells, similar to the pattern seen in humans, and expression of KIR2DL4, KIR3DS1, and KIR2DL5 by splenic NK cells. These observations indicate that linked regulatory elements within the genomic clones are sufficient to allow appropriate expression of KIRs in mice, and illustrate that the presence of the natural ligands for these receptors, in the form of human MHC class I proteins, is not necessary for the expression of the KIRs observed in these mice.

Alternative mRNA splicing creates transcripts encoding soluble proteins from most LILR genes

Leucocyte Ig‐like receptors (LILR) are a family of innate immune receptors expressed on myeloid and lymphoid cells that influence adaptive immune responses. We identified a common mechanism of alternative mRNA splicing, which generates transcripts that encode soluble protein isoforms of the majority of human LILR. These alternative splice variants lack transmembrane and cytoplasmic encoding regions, due to the transcription of a cryptic stop codon present in an intron 5′ of the transmembrane encoding exon. The alternative LILR transcripts were detected in cell types that express their membrane‐associated isoforms. Expression of the alternative LILRB1 transcript in transfected cells resulted in the release of a soluble ∼65 Kd LILRB1 protein into culture supernatants. Soluble LILRB1 protein was also detected in the culture supernatants of monocyte‐derived DC. In vitro assays suggested that soluble LILRB1 could block the interaction between membrane‐associated LILRB1 and HLA‐class I. Soluble LILRB1 may act as a dominant negative regulator of HLA‐class I‐mediated LILRB1 inhibition. Soluble isoforms of the other LILR may function in a comparable way.

Non-classical immunology

A report on the British Society for Immunology Annual Congress, Harrogate, UK, 5-8 December 2000.

Chapter 8 – Genetic Diversity in NK and NKT Cells

This chapter provides an overview of genetic diversity in natural killer (NK) and NKT cells. NK cells are large granular lymphocytes capable of lysing targets that have lost surface expression of major histocompatibility complex (MHC)-1. Since NK cells can provide immediate defense against infection, they are regarded as agents of the innate immune system. Like their MHC ligands, some of the “NK receptors” exhibit a high degree of genetic variation and appear to be evolving rapidly. When an NK cell is stimulated by a target, its subsequent response is dependent on a delicate balance between signals from activating and inhibitory receptors. Residues within the cytoplasmic domain determine the immunomodulatory effects of any given NK receptor. The focus of the chapter is genetic variations identified in MHC-1 specific NK receptors and their ligands in human and mouse systems. The potential implications of these variations and their relevance to immune mediated disease are discussed. Basics of NK receptor complexes and lectin receptors are focused on.

Wolbachia-induced reproductive isolation

Infection with the bacterium Wolbachia is sufficient to establish a reproductive barrier between two otherwise compatible species of wasp, making Wolbachia a potential driving force in evolution.