CD19 and systemic lupus erythematosus: BDCA2, blood dendritic cell antigen 2; CAR, chimeric antigen receptor; CD19, cluster of differentiation 19; CRS, cytokine-release syndrome; DNA, deoxyribonucleic acid; ICANS, immune effector cell-associated neurotoxicity syndrome; SLE, systemic lupus erythematosus; TLR7, toll-like receptor 7