Infection of CD8 lymphocytes by HIV-1 in the aetiology of AIDS

Despite many decades of research, how exactly HIV causes AIDS is not fully understood. In AIDS, the immune system is profoundly damaged by HIV, leaving the infected person susceptible to a range of infections and sometimes cancers that may be fatal if untreated. It is known that CD4 (or T-helper) lymphocytes are a major target for HIV in the infected person. This is because this cell type expresses on its cell surface the CD4 protein that HIV needs to attach and ultimately penetrate and infect the cell. Infection and destruction of CD4 lymphocytes is extremely damaging to the immune system, because this cell type plays a key role in various types of immune responses, such as antiviral cytotoxic T cell activity and antibody production (hence its name as a ?helper? cell). Recently, we have found that another type of lymphocyte, the CD8 lymphocyte, may also be infected by HIV. This cell type is a key part of the immune system?s response to infections, and in laboratory conditions, has been shown to become vulnerable to infection when reacting to a foreign target, such as an invading microbe or virus. From the data collected so far, infection and destruction of this cell type by HIV may be an important additional cause of the collapse of the immune system in AIDS.

When HIV infects a cell, the process of virus replication generates large numbers of new infectious viruses that bud from the cell surface to infect further cells. During this budding process, HIV may acquire a number of cellular proteins, often accidentally, sometimes deliberately. The type of cellular proteins found in virus envelopes therefore can show what cell type was infected. For example, HIV frequently contains a cell surface protein called CD36, a protein that is only found in macrophages. We have found that CD8 may also be frequently incorporated by viruses infecting CD8 lymphocytes. Using these cellular proteins as markers, we can analyse virus in plasma of infected individuals to determine which cells were infected. We can therefore determine to extent to which CD4, CD8 and macrophages are targeted by HIV, and which is most important infected cell type at various stages of HIV infection, ranging from acute infection through to AIDS. The use of this new method to study virus replication in infected individuals will be of importance investigating how individuals with AIDS become unable to control HIV and other infections.

The underlying mechanisms of immunodeficiency in AIDS still remain only partly understood. Although infection and destruction of CD4 lymphocytes plays a major part, chronic immune system activation and infection of other cell types such as macrophages play further roles in immune compromise and HIV-associated disease. We have additionally found CD8 lymphocytes are targeted by HIV though expression of CD4 after antigenic stimulation; consequent loss of cytotoxic T cell responses may represent further key elements in HIV-induced immunodeficiency. Investigating the extent of infection and resulting functional defects from infection of activated CD8 lymphocytes in vivo has been hampered by intrinsic difficulties in accessing this population using peripheral blood. However, we have recently developed a method of immunophenotyping cell-free HIV in plasma through their incorporation of cellular proteins during budding from cells. Thus, CD3, CD8 and CD36 act as selectable markers for bead or flow cytometry sorting methods, enabling the relative contributions of infected macrophages, CD8 and other lymphocytes (ie. CD4) to be determined. A cross-sectional survey of pre-treatment HIV-infected individuals demonstrated that infected CD8 lymphocytes made substantial contributions to the replicating population of HIV in vivo, findings with profound implications for understanding the nature of HIV-associated immunodeficiency.

Virion sorting will be used to explore cellular targeting during untreated infection, using plasma samples collected from large well defined groups during the period of acute viraemia during primary infection, throughout pre-symptomatic period, and into AIDS, and during treatment interruption. This study will enable cellular targeting in gastrointestinal tract-associated lymphoid tissue (GALT), depleted during primary infection, to be determined, and the potential switch from CD4 to CD8 lymphocytes as the former become depleted during AIDS progression. The method will be extended to characterise anatomically sources of virions infecting CD4 and CD8 lymphocytes based on their incorporation of lymph node and GALT homing markers, e.g. CD62L and CCR9, and enable the contribution of infection of GALT to the replicating population of HIV throughout the course of HIV infection to be assessed. Virion sorting based on incorporation of several cellular proteins will enable much finer resolution of cellular targeting by replicating virus than possible by other methods. We further propose to investigate the antigenic specificity of HIV-infected CD8 lymphocytes; over-representation of infected CD8 lymphocytes with anti-HIV specificities (as is the case for CD4 lymphocytes) would provide a new paradigm for investigating the nature of HIV-induced immunodeficiency and its inability to control virus replication.