Características de los factores de riesgo

1.4 T h e an tigen p ro cessin g com p artm en t

F o l l o w i n g effect iv e capture and in ter na lis at io n of exo gen ou s antigen, the s ec ond role that an APC must un de rt ak e in order for the T cell p o pu la ti o n to screen such material is the specific class I l- m ed ia te d deg ra d ati on of an ti ge ni c material (Shimo nk evi tz et al, 1983; La nza vec chi a, 1996). Much is k n o w n about the nature of the peptid es that bind to both class I and class II M H C molecules. However, unlike class I-mediated proc ess in g, the exact i n t r a c e l lu l a r location of class I l-m ed ia te d proc ess in g and pep ti de bin di ng of such proc ess es remains largely undefined.

The c om p ar tm e n t( s ) into which endoc yt os ed antigen is deli ver ed f ac i l it a t e ( s ) the generation of antigenic peptide fragme nts by a series of ev en ts c o m pr is in g reduction, déna tu ra ti on and limited prote oly si s (Zeigler & U n an ue , 1982; Streicher et al, 1984; Chain et al, 1988; Je nsen, 1991). Such pr oc es se s are co llectively termed antigen pr o ce ss in g and re sul t in the ge ne r a t i o n of peptides of 8-12 or more amino acids in length (Chain et al,

1988). The exact nature of the structure(s) in which such events occur is not ho w ev er , clearly defined as yet, and remains the subject of muc h study and debate. The fact that partial pro teolys is, ra ther than more subs ta nti al de g r a d a t i o n , is re quired for the gener ation of such pro du cts, im m ed ia te ly s ug ges ts that the lysosome is not the ideal co m p a r tm e n t for antigen p r o c e s s i n g (Chain et al, 1988). The lysosomal en v ir o n m e n t is too ag g r e ss iv e a co mp ar tm en t for lim ited degradation; its pr imary func tio n be i n g in the proteolytic destruction of cel lular debris. However, a num ber of stud ie s have pr odu ced data su gg esting that lysos omal p ro ce ss in g is

r eq ui red for the generation of im mu n o ge n ic fragments in both B cells and DC (Collins et al, 1991; Harding et al, 1991; Har ding et al, 1992; Har ding & Geuze, 1993; Kleijm eer et al, 1995). In addition, red uctio n of di su lp h id e bridges, a re qui rem ent for successful antigen p r oce ssi ng , occurs opti ma ll y un d er lysosomal conditions (Collins et al, 1991).

It is now accepted that antigen pr oce ss in g and the s ub seq uen t p ep t i d e- M H C II as so cia ti o n occurs within the endocytic pathw ay of APC (Gua gli ard i et al, 1990; Tulp et al, 1994; Barnes & Mitchell, 1995; K lei jm eer et al,

1995). The endocytic pathway itself is ub iq ui to us, and in light of the fact that only a small number of highly diffe re ntiat ed, s pe ci alis ed cells are cap ab le of class H-restricted antigen pr oce ss in g and pr es en ta ti on , one must assu me that such cells, i.e. APC, contain novel c o m p ar tm e nts along the endo cytic route which facilitate such a function. Such a structu re (s) is th o u g h t to be located at a point between the MHC II bi os yn th es is route and the endocytic pathway.

St udies conce rnin g the identi fic ati on of the antigen p ro ces si n g c o m p a r t m e n t have utilised both human and mu rine B cells as models (G uag li ard i et al, 1990; Harding & Geuze, 1993; C al af at et al, 1994; We st

et al, 1994; A m ig ore na et al, 1995; Barnes & M itchell, 1995; Peters et al,

1995). In such investigations, two candid ate in tra c el lu la r antigen p ro ce ss i n g com par tm en ts, each with ch ar act er ist ics distin ct from e n do s om e s or lysosomes have been identified; the MHC or MHC II- co nt a i n i n g co m p ar tm en t in human B cells, and the CHV, or class II co nt a i n i n g vesicle in murine B cells. As the n o m en cl at ur e suggests, the mai n feature com m on to both comp ar tm en ts is their high MHC II content. C o m p a r t m e n ts r es em bl in g the MHC have also been des cri bed in mu rine sp leni c DC (Kle ijmee r et al, 1995). All of the ca nd id ia te structures d es c ri b e d are acid in pH and contain p ro ces si ng enzymes (G uagliardi et al.

1990; Barnes & Mitchell, 1995; Klei jm eer et al, 1995). Whil e d ist inc t from the clas sical end ocytic comp ar tmen ts th emselves, the the CIIV appears to be related to an early endosomal structure, while the MIIC com pr ise a he t er ol ogo us set of vesicular structures, re se m bl in g c o m pa r tm e n ts from early to late en d o s om e (Calafat et al, 1994; Am ig o re n a et al, 1995; Barnes & Mitchell, 1995; Peters et al, 1995). Evidenc e exists to suggest that the fo rma ti on of the MIIC is induced by the presence of MH C II m ol ecu les , and that bio sy nt h es is of such molecules is re quired to mai nt ain the presence of MIIC c o m p a r tm e n ts (Calafat et al, 1994). As such, the MIIC appears to be highly s pe ci al is ed com partment, whose sole fu nction is in the loading of p r oce sse d antigen to MHC II molecules.

The hyp othesis suggested by Peters et al (1995) is the most co nv in ci n g to date, and p re se nt s a more bel ievable scenario than that of most, in that the burden of antigen pr ocessing and peptide loading to MHC II m ole cul es has been ascribed to a het erologous set of vesic ula r class II con ta in in g com p ar tm e nts , as opposed to one distinct c o m p o ne nt of the endocytic pathway. D if fe r e n t antigen and different epitopes within the same antigen may well re qu ire different proteolytic env ir o nm en ts for pr oce ssi ng and as such, it seems unlikely that a single c o m pa r tm e n t is r es po n s ib le for such a task for every d et er m in a nt within every en doc yt os ed antigen. Once in duced by the pr ese nce of MHC II molecules, antigen co nt ai ni n g MHC vesicles move towards a more mature, late endo so ma l nature. As such, the pr oteolytic e n v i r o n m e n t and MHC II mat ura tio n state will vary from vesicle to ves icl e in MHC (Peters et al, 1995). D i s cu ss ed in more detail in

section 1.8, the generation of different epit opes within the same, or