To ascertain w hich stages o f TN thym ocyte developm ent are dependent upon yc chain
expression, irradiated chim aeric mice treated w ith anti-yc chain m onoclonal antibodies,
w hich have a phenotype that closely resembles yc chain deficient mice, were analysed. Intrathym ic injection o f purified subsets o f TN thym ocytes at different stages o f developm ent into these mice failed to reconstitute T cell developm ent indicating that the yc chain is required for differentiation and grow th at all stages o f differentiation o f TN
pro-T cells (F ig l.2 ) (He et al., 1997). TN thym ocyte developm ent is arrested as CD 44‘*“"CD25’^ cells in yc chain deficient mice suggesting that the stage 2 transition is
the m ost yc chain dependent (Fig 1.2).
Table 1.2: The phenoty pe o f mice deficient in yc chain dependent cytokines or
receptors
K nockout mouse Phenotype
yc chain T and B cell developm ent impaired, absence o f lELs, TCRyÔ T cells and N K cells, age related accum ulation o f ‘activated’ CD4+ T cells (Cao et al., 1995; DiSanto et al., 1995)
IL-2 N orm al T, B and N K cell development, autoim m unity, colitis, myeloproliferative disease (W einburg and Parkman, 1990)
IL-4 N orm al T and B cell developm ent, im paired Th2-type responses (Kahn et al., 1991)
IL-2/IL-4 Sim ilar to IL2 and IL-4, with enhanced T-cell proliferation (Sadlack et al., 1994)
IL-2RP Sim ilar to IL-2, absence o f C D 8 a a lEL and NK cells, peripheral T and B cell proliferation (Suzuki et al., 1997)
IL-7 Impaired T and B cell developm ent, normal NK cells, absence o f TCRyÔ T cells (von Freeden Jeffry et al., 1995)
IL -7 R a Sim ilar to IL-7, with more severe block in T and B cell developm ent (Peschon et al., 1994)
yc chain/c-kit Sim ilar to yc chain, with complete lack o f thym ic development (Rodewald et al., 1997)
Animal models o f cytokine and cytokine receptor deficiencies suggest that the major contributor to the block in T cell developm ent in XSCID is the survival defect due to IL-7/IL-7R deficiency. IL-7 and IL -7R a deficient mice have severe lym phopenia in
peripheral blood and lym phoid organs and show similar lym phocyte abnorm alities to yc chain-deficient mice (Peschon et al., 1994; von Freeden Jeffry et al., 1995). D evelopm ent as far as CD 4'°CD44+CD25-c-kit+ thym ocytes in IL -7 R a deficient mice
is relatively independent o f IL -7R a signalling (F ig l.2). H owever, IL-7/IL-7R is im portant in the stage 1 transition, with the accum ulation o f CD 44+CD 25- cells due to a partial differentiation arrest at this stage. IL -7 R a is also involved in the TCRp chain rearrangem ents that occur during the stage 2 transition. The few peripheral T cells that do develop in IL-7 deficient mice respond normally to m itogenic stim ulation, whereas
those from IL -7 R a deficient mice show decreased proliferation and enhanced apoptosis (von Freeden Jeffry et al., 1995; M araskovsky et al., 1996). The more severe
phenotype in IL -7 R a deficient mice com pared to IL-7 deficient mice is attributed to the
utilisation o f the IL 7 R a chain in the TSLP receptor complex.
The T cell deficiencies resulting from yc chain and IL -7/IL -7R a deficiencies are not
equivalent, and the m ore severe block in T cell developm ent arising from yc chain deficiency m ay be due to the inhibition o f another at present unknow n cytokine that uses yc chain in the receptor complex. It is also som ew hat paradoxical that the IL-7/IL-
7 R a deficiency acts earlier in thym opoiesis, albeit less severely.
A significant role for yc chain signalling via the IL-7R in T lym phopoiesis appears to be the prevention o f cell apoptosis, rather than cell differentiation and/or proliferation, yc
chain-dependent signals are known to induce Bcl-2 expression, a proto-oncogene that prevents or delays cell death induced by radiation, glucocorticoids and cytokine w ithdraw al. The absence o f Bcl-2 expression regulation in yc chain-deficient mice was dem onstrated by N akajim a et al. (1999), with reduced levels in immature TN thym ocytes and im mature single positive (SP) thym ocytes although normal levels were observed in CD4+CD8+CD3'°" thym ocytes. Enforced expression o f the Bcl-2 transgene in all developm ental stages o f haem atopoietic cells in yc chain as well as IL- 7R deficient mice, resulted in substantial restoration o f T lym phopoiesis, underscoring the principal function o f IL-7 in cell survival (Akashi et a i , 1997; Kondo et al., 1997).
As the yc chain is involved in the IL-7R com plex and IL-7 is a cofactor for TCR
rearrangem ent, TCR transgenic yc chain-deficient mice were created to investigate the effect on thym ic cellularity o f bypassing this TC R rearrangem ent stage. Strong increases in cellularity were observed in m oderately high TCR/M H C affinity situations with a w eak increase seen in the high affinity TCR/M H C case (N akajim a et al., 1999). These results suggest that the yc chain is m ore im portant in the developm ent and/or survival o f T cells expressing TCRs with a relatively high affinity for self M HC/peptide com plexes rather than TCRs w ith a m oderate affinity for self M HC/peptide. W hen TCR and Bcl-2 transgenes are co-expressed in yc chain-deficient mice, they synergistically rescued T cell developm ent in a TCR/M H C dependent m anner as well, with a greater survival inducing effect from BcI-2 expression observed in the high affinity background (N akajim a et al., 1999). Thym ic cellularity is still dim inished as
com pared to w ildtype suggesting that the yc chain has additional functions beyond TCR rearrangem ent and BcI-2 induction, w hich requires further investigation. The dim inished cell division seen in yc chain-deficient mice may indicate a proliferation
defect.
Patients w ith defective IL -7 R a expression suffer from sim ilar T cell defects to those observed in hum an XSCID and defective IL-7 signalling may account for the major T cell abnorm alities in XSCID as well as in yc chain-deficient mice (Puel et al., 1998). Highly purified hum an CD34+ fetal liver stem cells differentiate to mature T cells when seeded in vitro into isolated fetal thymic lobes o f scid(-/-) m ice followed by fetal thym us organ culture (FTOC). Treatment with neutralising monoclonal antibodies (mAb) tow ards mouse IL-7, human IL-7 or human IL -7 R a resulted in a profound reduction in hum an thym ic cellularity and a block in early T-cell developm ent during the CD34+ precursor cells to immature CD 4+CD 1+CD 3- cells transition (Plum et al..
1996). These results indicate that IL-7 is indispensable in early human T cell development.
It is thought that prior to the CD 44+CD25+ stage other growth factors such as SCF may act in concert with the yc chain in the maintenance o f early thym ocyte precursors, yc chain and c-kit (stem cell factor (SCF) receptor) signalling pathways have synergistic effects on thym ocyte development. This was dem onstrated by the earliest and most severe thym ocyte-specific developmental block seen thus far in c-kifyc chain' mice,
whereas c-kit or yc chain-deficient mice have reduced thym ic cellularity but not a com plete block (Rodew ald et al., 1997).
IL-2 and IL-4 are not essential for early lym phocyte developm ent as T cell development is normal in IL-2 deficient patients and mice, IL-4 deficient m ice and in IL-2 & IL-4 deficient mice (W einburg and Parkman, 1990; Kahn et al., 1991; Sadlack et a l , 1994). However, targeted deletion o f IL-2 in the m ouse disturbs the process o f peripheral lym phoid apoptosis mediated by the Fas antigen, so that more extensive proliferation
occurs, with the accum ulation o f peripheral T C R a p T cells o f an activated phenotype (K neitz et al., 1995). This accum ulation o f activated T cells in the periphery is also
establishm ent o f antigen-specific unresponsiveness or anergy, w hich occurs when T cells are stim ulated through their antigen receptor w ithout the requisite co-stimulation (Theze et al., 1996). Therefore, T cells in these IL-2 deficient m ice m ay not receive the negative signals allow ing term ination o f the im m une response. A variety o f autoim m une m anifestations are provoked w ith inflam m atory bowel disease com parable with ulerative colitis in humans.