Peripheral B Cells from Patients with MGUS and Multiple Myeloma (MM) Can Be Stimulated by Paraprotein-Target Specific T-Helper Cells to Produce Paraprotein-Identical Monoclonal Antibodies: Rationale for PARs, a Novel Therapeutic Approach with Ultimate Specificity in MGUS/MM

Background: Paratarg-7 (P-7) is the antigenic target of paraproteins(Preuss et al. Int J Cancer 2009;125:656-61) from 15% of European and 37% of African-American MGUS/MM patients, stronlgy supporting a role of P-7 in the pathogenesis of MGUS/MM via chronic auto-antigenic stimulation. All patients with P-7 specific paraproteins are carriers of the hyperphosphorylated version of p-7 (pP-7). We recently identified pP-7 specific T-helper cells which were restricted by certain "permissive" HLA-DR haplotypes (Neumann et al., Int J Cancer 2015; 137:1076-1084). These HLA-DR subtypes are overrepresented among patients with P-7 specific paraproteins compared to the corresponding normal population indicating that there are two prerequisites for the development of MGUS/MM with a P-7 specific paraprotein: 1^st carriership of pP-7 and 2^nd a permissive HLA-DR subtype. We now investigated the functional role of the pP-7 specific T-helper cells and their interaction with peripheral B cells with cognate specificity.

Methods: Three patients with MGUS or MM, respectively, with a P-7 specific paraprotein and pP-7 specific T-helper cells were included in this study so far. In addition, the B cells from one healthy pP-7 carrying son of one of the patients were also analyzed. In vitro stimulation of antigen-specific peripheral B cells by pP-7 specific T-helper cells followed a modified protocol previously described by Lanzavecchia et al. (Eur J Immunol. 1983; 13:733-738). To this end, CD19⁺ B cells and CD3⁺ T cells were magnetically isolated from the proband's PBMC. T cells were replaced by corresponding T-helper cell clones.

Results: In all patients studied, the autologous pP-7 specific T-helper cells stimulated the peripheral B cells to produce P-7 specific antibodies. The P-7 specific B-cell responses were monoclonal and the immunoglobulin type was the same as the paraprotein of the corresponding patient. In contrast, B-cell stimulation with CMV-pp65 specific T-helper cells used as controls always induced an antigen-specific, yet polyclonal response. When the peripheral B cells of a pP-7 carrying patient's son were also stimulated with pP-7 specific T-helper cells, they induced - in contrast to the mother - a polyclonal P-7 specific antibody response in his B cells, even though mother and son shared a "permissive" HLA-DR haplotype (HLA-DRB1*1301).

Conclusion: In patients with MGUS/MM monoclonal B cells are found in the peripheral blood that can be induced to produce monoclonal antibodies identical to the serum paraprotein by T-helper cells with specificity for the antigenic target of the paraprotein. This does not only support an indispensable role of these T-helper cells in the pathogenesis of MGUS/MM via chronic antigenic stimulation, it also proves that precursors of the malignant plasma cells can be found in the peripheral blood that might fuel the development of malignant plasma cells. Cytogenetic and molecular genetic analyses are underway to determine if these precursor B-cells share the malignant genotype of their malignant plasma cells. These B cells can now be targeted by PARs (p araprotein a ntigens for r everse targeting) conjugated to toxins, as parts of bispecific constructs (PAR/CD3 or PAR/CD16) and/or PAR/CAR T cells. Use of PARs can be envisaged prophylactically for carriers of modified autoantigens like pP-7 with a permissive HLA-DR haplotype and a monoclonal B-cell response in vitro or in MM patients achieving a VGPR or CR after treatment for the prevention of relapse.