The technology relevant to multi-epitope (“minigene” e.g., “epigene” vaccines is developing. Several independent studies have established that induction of simultaneous immune responses against multiple epitopes can be achieved. For example, responses against a large number of T cell specificities can be induced and detected. In natural situations, Doolan et al (Immunity, Vol. 7(1):97-112 (1997)) simultaneously detected recall T cell responses, against as many as 17 different P. falciparum epitopes using PBMC from a single donor. Similarly, Bertoni and colleagues (J Clin Invest, Vol. 100(3):503-13 (1997)) detected simultaneous CTL responses against 12 different HBV-derived epitopes in a single donor. In terms of immunization with multi-epitope nucleic acid vaccines, several examples have been reported where multiple T cell responses were induced. For example, minigene vaccines composed of approximately ten MHC Class I epitopes in which all epitopes were immunogenic and/or antigenic have been reported. Specifically, minigene vaccines composed of 9 EBV (Thomson et al., Proc Natl Acad Sci USA, Vol. 92(13):5845-9 (1995)), 7 HIV (Woodberry et al., J Virol, Vol. 73(7):5320-5 (1999)), 10 murine (Thomson et al., J Immunol, Vol. 160(4):1717-23 (1998)) and 10 tumor-derived (Mateo et al., J Immunol, Vol. 163(7):4058-63 (1999)) epitopes have been shown to be active. It has also been shown that a multi-epitope DNA plasmid encoding nine different HLA-A2.1- and A11-restricted epitopes derived from HBV and HIV induced CTL against all epitopes (Ishioka et al., J Immunol, Vol. 162(7):3915-25 (1999)).
Thus, minigene vaccines containing multiple MHC Class I and Class II (i.e., CTL and HTL) epitopes can be designed, and presentation and recognition can be obtained for all epitopes. However, the immunogenicity of multi-epitope constructs appears to be strongly influenced by a number of variables, a number of which have heretofore been unknown. For example, the immunogenicity (or antigenicity) of the same epitope expressed in the context of different vaccine constructs can vary over several orders of magnitude. Thus, there exists a need to identify strategies to optimize multi-epitope vaccine constructs. Such optimization is important in terms of induction of potent immune responses and ultimately, for clinical efficacy. Accordingly, the present invention provides strategies to optimize antigenicity and immunogenicity of multi-epitope vaccines encompassing a large number of epitopes, and optimized multi-epitope vaccines, particularly minigene vaccines, generated in accordance with these strategies.
The following paragraphs provide a brief review of some of the main variables potentially influencing minigene immunogenicity, epitope processing, and presentation on antigen presenting cells (APCs) in association with Class I and Class II MHC molecules.
Immunodominance
Of the many thousand possible peptides that are encoded by a complex foreign pathogen, only a small fraction ends up in a peptide form capable of binding to MHC Class I antigens and thus of being recognized by T cells. This phenomenon, of obvious potential impact on the development of a multi-epitope vaccine, is known as immunodominance (Yewdell et al., Annu Rev Immunol, 17:51-88 (1999)). Several major variables contribute to immunodominance. Herein, we describe variables affecting the generation of the appropriate peptides, both in qualitative and quantitative terms, as a result of intracellular processing.
Junctional Epitopes
A junctional epitope is defined as an epitope created due to the juxtaposition of two other epitopes. The new epitope is composed of a C-terminal section derived from a first epitope, and an N-terminal section derived from a second epitope. Creation of junctional epitopes is a potential problem in the design of multi-epitope minigene vaccines, for both Class I and Class II restricted epitopes for the following reasons. Firstly, when developing a minigene composed of, or containing, human epitopes, which are typically tested for immunogenicity in HLA transgenic laboratory animals, the creation of murine epitopes could create undesired immunodominance effects. Secondly, the creation of new, unintended epitopes for human HLA Class I or Class II molecules could elicit in vaccine recipients, new T cell specificities that are not expressed by infected cells or tumors that are targets of induced T cell responses. These responses are by definition irrelevant and ineffective and could even be counterproductive, by creating undesired immunodominance effects.
The existence of junctional epitopes has been documented in a variety of different experimental situations. Gefter and collaborators first demonstrated the effect in a system in which two different Class II restricted epitopes were juxtaposed and colinearly synthesized (Perkins et al., J Immunol, Vol. 146(7):2137-44 (1991)). The effect was so marked that the immune system recognition of the epitopes could be completely “silenced” by these new junctional epitopes (Wang et al., Cell Immunol, Vol. 143(2):284-97 (1992)). Helper T cells directed against junctional epitopes were also observed in humans as a result of immunization with a synthetic lipopeptide, which was composed of an HLA-A2-restricted HBV-derived immunodominant CTL epitope, and a universal Tetanus Toxoid-derived HTL epitope (Livingston et al, J Immunol, Vol. 159(3):1383-92 (1997)). Thus, the creation of junctional epitopes is a major consideration in the design of multi-epitope constructs.
The present invention provides methods of addressing this problem and avoiding or minimizing the occurrence of junctional epitopes.
Flanking Regions
Class I restricted epitopes are generated by a complex process (Yewdell et al., Annu Rev Immunol, 17:51-88 (1999)). Limited proteolysis involving endoproteases and potential trimming by exoproteases is followed by translocation across the endoplasmic reticulum (ER) membrane by transporters associated with antigen processing (TAP) molecules. The major cytosolic protease complex involved in generation of antigenic peptides, and their precursors, is the proteasome (Niedermann et al., Immunity, Vol. 2(3):289-99 (1995)), although ER trimming of CTL precursors has also been demonstrated (Paz et al., Immunity Vol. 11(2):241-51 (1999)). It has long been debated whether or not the residues immediately flanking the C and N terminus of the epitope, have an influence on the efficiency of epitope generation.
The yield and availability of processed epitope has been implicated as a major variable in determining immunogenicity and could thus clearly have a major impact on overall minigene potency in that the magnitude of immune response can be directly proportional to the amount of epitope bound by MHC and displayed for T cell recognition. Several studies have provided evidence that this is indeed the case. For example, induction of virus-specific CTL that is essentially proportional to epitope density (Wherry et al., J Immunol, Vol. 163(7):3735-45 (1999)) has been observed. Further, recombinant minigenes, which encode a preprocessed optimal epitope, have been used to induce higher levels of epitope expression than naturally observed with full-length protein (Anton et al., J Immunol, Vol. 158(6):2535-42 (1997)). In general, minigene priming has been shown to be more effective than priming with the whole antigen (Restifo et al., J Immunol, Vol. 154(9):4414-22 (1995); Ishioka et al., J Immunol, Vol. 162(7):3915-25 (1999)), even though some exceptions have been noted (Iwasaki et al., Vaccine, Vol. 17(15-16):2081-8 (1999)).
Early studies concluded that residues within the epitope (Hahn et al., J Exp Med, Vol. 176(5):1335-41 (1992)) primarily regulate immunogenicity. Similar conclusions were reached by other studies, mostly based on grafting an epitope in an unrelated gene, or in the same gene, but in a different location (Chimini et al., J Exp Med, Vol. 169(1):297-302 (1989); Hahn et al., J Exp Med, Vol. 174(3):733-6 (1991)). Other experiments however (Del Val et al., Cell, Vol. 66(6):1145-53 (1991); Hahn et al., J Exp Med, Vol. 176(5):1335-41 (1992)), suggested that residues localized directly adjacent to the CTL epitope can directly influence recognition (Couillin et al., J Exp Med, Vol. 180(3):1129-34 (1994); Bergmann et al., J. Virol. Vol. 68(8):5306-10 (1994)). In the context of minigene vaccines, the controversy has been renewed. Shastri and coworkers (Shastri et al., J Immunol, Vol. 155(9):4339-46 (1995)) found that T cell responses were not significantly affected by varying the N-terminal flanking residue but were inhibited by the addition of a single C-terminal flanking residue. The most dramatic inhibition was observed with isoleucine, leucine, cysteine, and proline as the C-terminal flanking residues. In contrast, Gileadi (Gileadi et al., Eur J Immunol, Vol. 29(7):2213-22 (1999)) reported profound effects as a function of the residues located at the N terminus of mouse influenza virus epitopes. Bergmann and coworkers found that aromatic, basic and alanine residues supported efficient epitope recognition, while G and P residues were strongly inhibitory (Bergmann et al., J Immunol, Vol. 157(8):3242-9 (1996)). In contrast, Lippolis (Lippolis et al., J Virol, Vol. 69(5):3134-46 (1995)) concluded that substituting flanking residues did not effect recognition. However, only rather conservative substitutions that are unlikely to affect proteasome specificity were tested.
It appears that the specificity of these effects, and in general of natural epitopes, roughly correlates with proteasome specificity. For example, proteasome specificity is partly trypsin-like (Niedermann et al., Immunity, Vol. 2(3):289-99 (1995)), with cleavage following basic amino acids. Nevertheless, efficient cleavage of the carboxyl side of hydrophobic and acidic residues is also possible. Consistent with these specificities are the studies of Sherman and collaborators, which found that an R to H mutation at the position following the C-terminus of a p53 epitope affects proteasome-mediated processing of the protein (Theobald et al., J Exp Med, Vol. 188(6):1017-28 (1998)). Several other studies (Hanke et al., J Gen Virol, Vol. 79 (Pt 1):83-90 (1998); Thomson et al., Proc Natl Acad Sci USA, Vol. 92(13):5845-9 (1995)) indicated that minigenes can be constructed utilizing minimal epitopes, and that these flanking sequences appear not be required, although the potential for further optimization by the use of flanking regions was also acknowledged.
In sum, for HLA Class I epitopes, the effects of flanking regions on processing and presentation of CTL epitopes is as yet undefined. A systematic analysis of the effect of modulation of flanking regions has not been performed for minigene vaccines. Thus, analysis utilizing minigene vaccines encoding epitopes restricted by human Class I in general is needed. The present invention provides such an analysis and accordingly, provides multi-epitope vaccine constructs optimized for immunogenicity and antigenicity, and methods of designing such constructs.
HLA Class II peptide complexes are also generated as a result of a complex series of events that is distinct from HLA Class I processing. The processing pathway involves association with Invariant chain (Ii), its transport to specialized compartments, the degradation of Ii to CLIP, and HLA-DM catalyzed removal of CLIP (see (Blum et al., Crit Rev Immunol, Vol. 17(5-6):411-7 (1997); Arndt et al., Immunol Res, Vol. 16(3):261-72 (1997)) for review. Moreover, there is a potentially crucial role of various cathepsins in general, and cathepsin S and L in particular, in Ii degradation (Nakagawa et al., Immunity, Vol. 10(2):207-17 (1999)). In terms of generation of functional epitopes however, the process appears to be somewhat less selective (Chapman H. A., Curr Opin Immunol, Vol. 10(1):93-102 (1998)), and peptides of many sizes can bind to MHC Class II (Hunt et al., Science, Vol. 256(5065):1817-20 (1992)). Most or all of the possible peptides appear to be generated (Moudgil et al., J Immunol, Vol. 159(6):2574-9 (1997); and Thomson et al., J Virol, Vol. 72(3):2246-52 (1998)). Thus, as compared to the issue of flanking regions, the creation of junctional epitopes can be a more serious concern in particular embodiments.