QLS4131 Plus Isatuximab in Newly Diagnosed Primary Plasma Cell Leukemia

The goal of this clinical trial is to learn whether QLS4131, a BCMA/GPRC5D/CD3 trispecific antibody, can improve treatment outcomes when combined with isatuximab in adults with newly diagnosed primary plasma cell leukemia (pPCL). The study will also evaluate the safety of this treatment combination. The main questions it aims to answer are: * How many participants achieve measurable residual disease (MRD) negativity after 9 treatment cycles? * What side effects occur during treatment with QLS4131 combined with isatuximab? * How many participants respond to treatment, and how many achieve a complete response? * How many participants achieve sustained MRD negativity? * How long do participants remain free from disease progression, and how long do they survive after starting treatment? All participants will receive the study treatment. There is no comparison group in this study. Participants will: * Receive QLS4131 by subcutaneous injection together with intravenous isatuximab in 28-day treatment cycles. * Undergo regular blood tests, bone marrow examinations, and disease assessments to monitor treatment response and safety. * Undergo peripheral blood stem cell mobilization and collection after the first 3 treatment cycles, if appropriate. * Be monitored throughout the study for side effects, including adverse events and serious adverse events.