O b serv a tio n s like the ones described above led to a variety o f fu n c tio n s in a n tig en p re s e n ta tio n th at w ere ascrib ed to li: a c h a p e ro n e -lik e function in the folding and assem bly of class II m olecules; the inhibition of peptide binding in the ER by blocking the M H C class II peptide binding groove; prom otion o f egress of class II m olecules from the ER, targeting to the endocytic pathway and re te n tio n o f class II m olecules in endocytic com partm ents (Cresswell, 1992; W olf and Ploegh, 1995). With the generation o f li k n o c k -o u t m ice it b ecam e possible to assess the im portance o f th e s e d if f e r e n t fu n c tio n s. In v a ria n t c h a in -d e fic ie n t m ice w ere
found to have drastically reduced surface levels o f MHC class II m olecules. Instead o f travelling to the cell surface the class II m o lecu les in \i~l~ anim als are re ta in e d in the E R w here they associate with molecules o f the “ER quality control m achinery” like BiP. In spleen cells o f l i ’ / ' a n im a ls w ith H-2^ b a c k g r o u n d significantly increased num bers o f free a and p chains were found, suggesting a chaperone function for li in prom oting and stabilising M HC class II subunit association. H ow ever, this effect m ight be allele-d ep en d en t since studies with class II m olecules other than I - A ^ indicate that li is dispensable for the a /p s u b u n it in te r a c tio n (Miller and Germain, 1986; Sekaly et al., 1986; Bijlm akers et al., 1994; B ik o ff et al., 1995; V iville et al., 1993). T he li-d e fiç ie n t anim ais have vastly re d u c ed n u m b ers o f CD4+ T cells due to deficient positive selection by M HC class 11+ cells in the thymus. Closer exam ination of the class II m olecules that are expressed on the cell surface even in the absence of li showed that they had an ab erran t, S D S -u n sta b le c o n fo rm a tio n in d ic a tiv e o f the lack of tig h tly b in d in g p e p tid es. The su rfa ce class II m o le cu le s also exhibited enhanced binding o f peptides as if they were empty or occupied by easily displaced peptides (B ikoff et al., 1995; Viville et al., 1993). This observation was slightly surprising since it was generally assum ed that by blocking the peptide binding groove li prev en ted prem ature peptide binding to class II in the ER and m ain tain ed the segregation betw een the class I and the class II antigen presentation pathw ay (Teyton et al., 1990). Even though li does physically block the peptide binding groove o f class II in the ER the prevention of peptide binding at that point does not seem to be one o f its m ajor functions since according to the li ^ data peptides in the ER do not seem to be appropriate ligands for class
II. A n aly sis o f the li-n e g a tiv e m ice c o n firm ed th at li has an im p o rtan t fu n ctio n in the targ etin g o f class II m o lecu les to an e n d o cy tic p e p tid e -lo a d in g co m p artm en t: P re se n ta tio n o f fo re ig n antigens is severely im p aired in Ii“/ “ m ice (V iville et al., 1993), suggesting that in the absence o f li MHC class II m olecules do not reach the right compartments where antigens are degraded.
R ecently a novel function of li in the developm ent o f B cells has been suggested based on the o bservation th at im m atu re v irg in B c e lls in l i '/ " m ice d isp la y a d e v e lo p m e n ta l a rre s t th a t is in d ep e n d en t on M HC class II ex p ressio n (S h ach ar and F lav ell,
1996). O ne p o ssib le in te rp re ta tio n o f this fin d in g is th a t cell su rfa c e -e x p re s s e d li m ig h t c o n v ey d e v e lo p m e n ta lly im p o rta n t signals to B cells. C onceivably, the c h o n d ro itin -su lfa te m o d ified form of li that has been described earlier, could be o f im portance for this phenomenon.