*Purpose: BCL6, is a transcription factor involved in B cell activation and differentiation. BCL6-expressing B cells play a crucial role in the development and maintenance of germinal centers, which are essential for the development of a humoral response. Targeting BCL6-mediated responses has the potential to prevent humoral alloreactivity. Here, a small molecule BCL6 inhibitor named 79-6 was tested in vitro and its effect on plasma blast formation and IgG production was investigated.

*Methods: The following experiments were performed in the presence and absence of the small molecule BCL-6 inhibitor 76-9 (range 25-100 µg/mL): (1) Polyclonally-activated B cells (anti-IgM/anti-CD40 and IL-21) from healthy controls were studied for differentiation, plasma cell formation and IgG-production. (2) To study 79-6’s inhibitory effect on B cell differentiation stages, circulating TfH cells and B cells were stimulated with alloantigen, and 79-6 was added at different time points (day 0, 3, and 7).

*Results: After polyclonal stimulation, a median of 7.4% of the B cells differentiated into plasmablasts. In the presence of 79-6, plasmablast formation was significantly inhibited by 91% and the proportion of class switched memory B cells dropped by 22%, both p<0.01). Production of IgGs was measured in culture supernatants (median of 600 ng/ml), After inhibition by 79-6, IgG-concentrations were significantly reduced (91%, p <0.01). After stimulation with alloantigen, B cells successfully differentiated into plasma blasts (median 9.8%). Early addition of 79-6 (day 0, day 3) resulted in inhibition of plasma blast formation (median inhibition: 97% and 73%, respectively), while addition of 79-7 at day 7, when B cells have differentiated into plasmablast, did not result in significant inhibition of plasma blast formation.

*Conclusions: 79-6 effectively inhibits differentiation of B lymphocytes into immunoglobulin-producing plasmablasts, whereas it does not inhibit Ig production once plasmablast formation is established. This implies that the timing of 79-6 administration in clinical practice is crucial.

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