Cross-Reactive T_H1 Cells to Aspergillus Fumigatus and Candida Albicans

Abstract 1268

Aspergillus fumigatus and Candida albicans are the most common fungal pathogens causing severe invasive infections with substantial mortality in immunocompromised hosts. T_H1 immunity plays a pivotal role in the clearance of most fungal infections including invasive aspergillosis, and vaccination strategies as well as adoptive transfer of A. fumigatus-specific CD4⁺ T_H1 cells have already shown promising results in mouse models and adoptive transfer also in HSCT recipients. Previous data in mice have suggested the existence of cross-protective immunity between different fungal species mediated by antibodies recognizing fungal cell wall structures. However, T_H1-mediated cross-protective immunity would probably be more efficient to protect against different fungal infections. Our own data as well as at least 3 independent previous studies have shown the ability of the A. fumigatus extracellular cell wall glucanase Crf1 to elicit protective T_H1 immune responses in mice and humans. Due to sequence similarities between A. fumigatus Crf1 and its counterparts in other fungal species, Crf1-specific T_H1 cells could be able to mediate cross-protection to different fungal species and we therefore aimed to identify potent Crf1 peptide epitopes useful for clinical application.

To assess the potential of different Crf1 peptide epitopes to induce robust immune responses we generated T cell clones specific for 8 Crf1 peptides identified by epitope mapping and determined their reactivity to endogenously processed A. fumigatus antigens. Only T cell clones specific for 4 of the 8 epitopes were able to recognize and respond to mRNA-transfected dendritic cells as well as A. fumigatus extract and inactivated fungus. Testing of the T cell clones for cross-reactivity to other fungal species showed that all p41-specific T cell clones could not only be efficiently activated by A. fumigatus but were also highly reactive to C. albicans. In contrast, T cell clones specific for the other 3 functional Crf1 epitopes showed no reactivity to C. albicans. This can be explained by the fact that only the sequence of the p41 peptide is homologous in both species with only one amino acid difference whereas the sequences of all other peptides differ considerably. Furthermore all p41-specific T cell clones responded to all tested clinical isolates of both C. albicans and A. fumigatus but not to other Aspergillus or Candida species. The cross-protective p41 epitope has the further advantage of a rather broad MHC restriction and can be efficiently presented by at least 3 different MHC class II alleles, which cover more than 50% of the Caucasian population. The immune response to p41 is oligoclonal as nearly all tested donors displayed a diverse T cell receptor Vb repertoire. In order to translate these findings into clinical immunotherapy protocols (i) we established a protocol that allows expansion of Crf1-specific CD4⁺ T cells to frequencies ranging from 42–63% within 14 days despite undetectable precursor frequencies in healthy individuals and (ii) verified our fungal target Crf1 for cross-protection in a mouse model for both A.fumigatus and C.albicans. Within this system we could demonstrate that mice pre-infected with A. fumigatus or Crf1 protein were protected against subsequent rechallenge with C. albicans and vice versa.

In conclusion, the A. fumigatus Crf1 epitope p41 is able to induce robust T_H1 responses in the majority of healthy individuals and T cells specific for this epitope are highly reactive to both, A. fumigatus and C. albicans. This T cell-based cross-protection between the two fungal species can be observed in vitro in the human system as well as in vivo in mice. The p41 epitope might therefore be of great interest for the generation of vaccination strategies or adoptive T cell therapies which might be integrated in or replace antifungal therapy to minimise development of fungal resistance and drug interactions and toxicity.