Evaluation of Different Co-Stimulatory Signals in the Priming and Expansion of HLA-B*0702/CMV_pp65 Restricted CTLs after Stimulation with aAPC

The clinical application of T-cells for adoptive immunotherapy often requires the ex vivo stimulation and expansion of antigen-specific T cells and the re-transfer of the expanded T cells into patients. One strategy to generate such T-cell products includes the use of autologous antigen-presenting cells (APCs) which vary essentially in quality and quantity. Furthermore, other strategies for the generation of APCs such as dendritic cells (DCs) are labor intensive and expensive. To circumvent these disadvantages, many investigators have focussed their research on the development of artificial APCs (aAPCs). Recent studies have demonstrated that microbeads loaded with MHC-peptide molecules in monomeric or Ig-coupled dimeric forms and co-stimulatory antibodies such as anti-CD28 and anti-4-1BB are reproducibly capable to induce high numbers of antigen-specific T-cells. In our study, we wanted to address the question if the use of further stimulatory signals in addition to CD28/4-1BB could induce higher stimulatory capacities in aAPCs.

We loaded epoxy surface activated magnetic beads (DynaBeads Epoxy M-450) with HLA-B*0702/CMV_pp65 (TPRVTGGGAM) monomer and activating antibodies (AB) and/or ligands (L) against co-stimulatory molecules of the CD28 and/or the TNF-receptor (TNFR) family: namely CD28 (AB), ICOS (AB), 4-1BB (AB+L), CD27 (L), OX40 (L). The molar ratio between MHC and the co-stimulatory molecules was between 1:1 and 1:5 with a total amount of MHC-molecules of approximately 2×10⁵ molecules/aAPC. Cell cultures were maintained as described previously ¹ and analyzed weekly by staining with HLA-B*0702/CMV_pp65 (TPRVTGGGAM) tetramers and surface antibodies against the surface markers CD3, CD8, CD45RA, CCR7, CD57 and CD25.

Our results demonstrate that CD28 triggering is pivotal on aAPCs besides the antigen-specific signal for an optimal expansion of CMV-specific CTL. This signal can not be replaced by ICOS or 4-1BB either alone or in combination. The addition of ICOS and/or 4-1BB to CD28 did not significantly improve the percentage of specific T-cells after 3 weeks of culture although a trend to higher cell numbers and higher specificity when combining CD28, ICOS (AB) and 4-1BB (AB) was observed. Here, longer time periods may be required to evaluate the capacity of this combination to maintain long term cultures of antigen-specific CTL. Therefore, we always applied HLA-B*0702/CMV_pp65 monomer and CD28 (AB) on the aAPCs in further experiments and supplemented these molecules by the other co-stimulatory signals as described above.

Our data suggests that the phenotype (and thereby potential effector functions/in vivo survival) varies when triggering different costimulatory pathways via aAPCs. Interestingly, we could detect a higher amount of central memory T-cells (CCR7+/CD45RA−) in samples co-stimulated with the 4-1BB antibody, compared to samples treated with other co-stimulations [e.g. MHC/CD28: 6.2 % of CD8⁺/Tet⁺ vs. MHC/CD28/4-1BB (AB): 23.5 % of CD8⁺/Tet⁺]. Another difference was seen in OX40 ligand co-stimulated samples when analyzing CD57 and CD25 expression: Here, CD8⁺/tetramer⁺ T-cells had a higher content of non-activated (CD25⁻) T-cells with less replicative history (CD57⁻) than samples co-stimulated with other co-stimulations [e.g. MHC/CD28: 23 % of CD8⁺/Tet⁺ vs. MHC/CD28/OX40 (L): 45.2 % of CD8⁺/Tet⁺].

These preliminary findings suggest that the CD28 signal on aAPCs is indispensable for expansion of CMV-specific T-cells. But the phenotype and thereby in vivo functionality may be controlled by the use of defined other co-stimulatory signals like members of the CD28 and/or TNFR family. Further tests will have to define optimal amounts of MHC-molecules on aAPCs and their ratio to co-stimulatory molecules.