INTERPRETACIÓN DE LOS DATOS

Peripheral naïve T cells in young adults have a diverse repertoire o f TCRs (Arstilla 1999). T cell activation results in massive expansion o f cells with clonotypic TCR towards the encountered antigen (Butz 1998, Callan 1996). In most infections the immune response is directed by the expansion o f dominant clones, although subdominant clones are also involved. EBV infection induces a strong CD8^ T cell response. The expansion o f cells is largely oligoclonal based on dominant epitopes such as GLCTLVAML firom the early lytic cycle antigen BMLFl (Annels 2000) and FLRGRATGL (Callan 1998). The expansion of FLRGRATGL epitope specific cells are seen both early and late in the immune response. The extent o f expansion o f clones is dependent on antigen dose, duration o f exposure and cytokine environment (Gallimore 1998, Butz 1998, Tough 1996, Hou 1994), the timing o f recruitment o f clones is also an important factor in the composition o f the response (Bousso 1999). Expansions of cells and their differentiation into memory populations results in a more restricted TCR repertoire in memory and effector populations (Arstilla 1999). The clonality o f an individual may reflect past immunological history, and also their ability to respond to neo-antigens.

Clonally restricted populations of human cells were first documented by Hingorani in 1993, with clonal predominance of some TCR found in CD8^CD45R0^ cells (Hingorani 1993). Since then a number of cellular and molecular techniques have detected expanded populations in humans and mice (Wack 1998, Ku 1997, Schwab 1997, Posnett 1994, Callahan 1993). Many studies have shown that the presence o f large monoclonal expansions correlates with increasing age (Wack 1998, Schwab 1997, Posnett 1994); this reflects the immunological history o f donors and the reduced ability o f elderly individuals to respond to neo-antigens. Expansions are commonly observed within CD8^CD45R0^ populations of elderly individuals (Bernardin 2003, Shen 1998) and can also be detected in these populations at a lower frequency in young individuals. Clonal expansions in CD45RA^, naïve, populations are less frequent reflecting the antigen-inexperience of these cells. However, clonal expansions have been detected at a lower frequency in this population in elderly individuals (Wack 1998) and are occasionally seen in young donors (Maini 2000). Clonal expansions in CD45R0^ cells and in postulated ‘revertant’ CD45RA^ cells have been detected in viral infections including EBV, CMV and HIV (Wills 1999, Kern 1999, Maini 2000) further implying that oligoclonality of cells correlates to immunological history. Many expansions in CD45RA^ cells have been found to be CMV specific (Wills 1999). Clonal expansions have been observed over a number o f years implying that they are representative o f stable memory populations (Schwab 1997).

Detectable expansions in CD4^ cells are less frequent and, ex vivo, are usually only seen in centenarians (Bernardin 2003, Wack 1998). Antigen specific clones are present in some defined CD4'*’ populations, and have been detected in diseases including EBV (Carmichael, oral presentation, Jenner Institute CD4^ T cell memory meeting, October

Chapter 1 : Introduction 2002) and rheumatoid arthritis (Wagner 2003). Previous studies o f the activation o f CD4^ T cells in mice have shown that the duration o f acute responses is shorter than that o f CD8^ cells (Doherty 1996), and although there are expansions o f EBV specific CD4^ cells, this expansion is much smaller than the expansion o f CD8^ cells (Miyawaki 1991). These data suggest that CD4^ and CD8^ cells responses are controlled differently. The low frequency of detection of CD4^ clones reflects a more polyclonal repertoire o f TCR (Wang 2001, Li Pira 1998). This may be due to a number of influences. 1. The cells responding may consist of smaller expansions of a more diverse repertoire. 2. The expansion phase may be more tightly regulated and prevent clonal expansions reaching detectable levels. 3. The expansion o f cells may be as large as that seen in CD8^ cells, but with a larger contraction phase in CD4^ cells, leaving them undetectable except in the acute expansion phase (Maini 1999). In healthy, young individuals expansions are only observed following in vitro restimulation with recall antigens (Maini 1998, Li Pira 1998). The clonality o f CD4^ populations is discussed in more detail in Chapters 4 and 5.

The observation of oligoclonal expansions does not necessarily correlate with antigen specificity. However, comparisons of ex vivo CD4^ T cells to in vitro tetanus toxoid stimulated cells fi*om a donor recently vaccinated with TT showed that some ex vivo

clones were antigen specific (Maini 1998). When studying clonality it is important to recognise that detectable and functional repertoires are different. Mechanisms o f central and peripheral tolerance generate cells that are anergic and lack the ability to respond to antigen. Anergic and tolerant cells maybe visualised by techniques used to study clonality if they occur in the total repertoire at detectable frequencies. However, these cells are not capable o f responding to antigen so are not part o f the functional

‘responding’ repertoire. The different roles of naïve and expanded populations o f cells must also be considered when issues o f clonality are addressed. Nonetheless, studies of clonality o f T cells have provided new insights into mechanisms o f immune responses.