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We propose a novel approach for developing privacy-preserving large-scale
recommender systems using differentially private (DP) large language models
(LLMs) which overcomes certain challenges and limitations in DP training these
complex systems. Our method is particularly well suited for the emerging area
of LLM-based recommender systems, but can be readily employed for any
recommender systems that process representations of natural language inputs.
Our approach involves using DP training methods to fine-tune a publicly
pre-trained LLM on a query generation task. The resulting model can generate
private synthetic queries representative of the original queries which can be
freely shared for any downstream non-private recommendation training procedures
without incurring any additional privacy cost. We evaluate our method on its
ability to securely train effective deep retrieval models, and we observe
significant improvements in their retrieval quality without compromising
query-level privacy guarantees compared to methods where the retrieval models
are directly DP trained.