The role of natural killer cells in host defence against varicella

We know that the lymphocytes of our immune system are very important for control of the virus that causes chickenpox and shingles, VZV. When immunity is suppressed, for instance by cancer chemotherapy, chickenpox may be a very severe illness. Occasionally, previously healthy children become similarly unwell, prompting us to ask if their immune system is faulty. Often, none of the known causes of inherited immunodeficiency can be found, but in rare cases there is a shortage of natural killer (NK) lymphocytes. This may be giving us a clue about how VZV is contained by the normal immune system.

In the first part of this project we will investigate whether and how NK cells could contribute to the control of VZV in normal individuals. We will then study antiviral immune responses in children who have suffered unusually severe chickenpox. If we are right that NK cells help to control VZV, we predict that some of these children will have inherited problems with NK function. We will try to identify which specific genes are faulty. Improved understanding of the way the immune system interacts with viruses like VZV will help us to look after a broader group of affected children and adults.

Understanding the concerted host immune response to varicella zoster virus (VZV) remains an important goal. Despite the availability of effective antiviral chemotherapy and a live attenuated vaccine against VZV, deaths and morbidity due to severe varicella continue to occur. Much of this burden is borne by previously healthy children who have no evidence of a general impairment of cellular immunity. Among this group, however, the presence of rare individuals with an absolute deficiency of natural killer (NK) cells points to a key role for this lymphocyte subset in protection against VZV.

The overall aim of this project is to discover how NK cells participate in host defence against VZV. Specifically, I will test the ability of NK cells to recognize VZV-infected targets and the outcome of their encounter. I will ascertain what molecular interactions mediate NK recognition of VZV-infected cells. Finally, I will test the hypothesis that inherited defects of NK function produce heightened susceptibility to varicella.

NK activity against VZV-infected cells will be studied in vitro using well-established techniques with which I am familiar. I will examine the ability of VZV-infected fibroblasts to elicit cytotoxicity and cytokine release by NK cell lines derived from healthy adult peripheral blood. I will probe the molecular basis of target recognition, by investigating the effect of blocking specific NK activating receptors. This will be complemented by an analysis of how VZV modulates the expression of putative NK ligands. Key findings will be confirmed in an autologous system. An important goal will be to extend these analyses to VZV-infected dendritic cells and thereby explore potential immunomodulatory outcomes of their interaction with NK cells.

Insights obtained in the first half of the fellowship will be related to real patients in the second, when I will study children who have suffered severe varicella. I will apply a panel of flow cytometric and functional assays of NK cells alongside measures of adaptive immunity to VZV. To further assist in the identification of potential disease-causing genes, the transcriptional profile of NK cells from patients and controls will be compared using microarray analysis. Candidate genes will be sequenced to identify disease-causing mutations. Exploration of their effects at cellular and molecular levels will form the basis of further experimental study.

These important investigations promise to shed light on early events in the pathogenesis of severe varicella, and may identify new targets for immunomodulatory therapy for this devastating illness.