A Study of CC312 for Relapsed/Refractory Autoimmune Diseases

Inclusion Criteria: * Fully understand the trial's purpose, nature, methodology, and potential adverse reactions, voluntarily participate as a subject, and sign the informed consent form. * Aged 18-65 years (inclusive, based on the date of signing the informed consent form), regardless of gender. * For Systemic Lupus Erythematosus (SLE): 1. Diagnosed with SLE according to the 2019 EULAR/ACR classification criteria； 2. Meet at least one of the following: positive antinuclear antibody (ANA) and/or anti-dsDNA antibody and/or anti-Sm antibody at screening； 3. Had an inadequate response or relapse after standard therapy, defined as any of the following (alone or combined): glucocorticoids, antimalarials (hydroxychloroquine), immunosuppressants (including mycophenolate mofetil, cyclophosphamide, leflunomide, methotrexate, tacrolimus, cyclosporine, azathioprine), or biologics (rituximab, belimumab, telitacicept). Each regimen must have been administered for ≥3 months, and the subject must have received ≥2 immunosuppressants and/or biologics； 4. At screening, meet SLEDAI-2000 ≥7 and have at least one BILAG A or two BILAG B organ domain scores； 5. Prior to the first dose, subjects must have received glucocorticoids and/or antimalarials and/or immunosuppressants for ≥12 weeks, with stable doses for ≥30 days； 6. If receiving oral glucocorticoids (e.g., prednisone), the dose must be ≤40 mg/day at screening and during the screening period； 7. If using glucocorticoids alone, the dose must be ≥7.5 mg/day prednisone (or equivalent). * For Idiopathic Inflammatory Myopathy (IIM): 1. Diagnosed with possible or definite IIM per the 2017 EULAR/ACR classification criteria (≥5.5 points without biopsy; ≥6.7 points with biopsy) ； 2. Have at least one positive myositis-specific autoantibody (MSA) , myositis-associated autoantibody (MAA), or ANA at or prior to screening； 3. Had an inadequate response or relapse after conventional therapy, defined as glucocorticoids (prednisone \>1 mg/kg/day or equivalent) and ≥1 immunomodulatory drug (immunosuppressants: azathioprine, mycophenolate mofetil, cyclophosphamide, methotrexate, leflunomide, tacrolimus, cyclosporine; biologics: rituximab, belimumab; small molecules: tofacitinib), each for ≥3 months； 4. Have active IIM at screening, defined as meeting ≥3 of MMT-8 total score ≤141/150 with ≥20% strength loss in affected muscles; Physician global activity ≥2; Patient global activity ≥2; myositis disease activity assessment tool (MDAAT) ≥2; ≥2 muscle enzymes elevated, with one ≥1.5× upper limit of normal (ULN); health assessment questionnaire (HAQ) ≥0.25； 5. Prior to the first dose, subjects must have received glucocorticoids and/or immunosuppressants for ≥12 weeks, with stable glucocorticoid doses for ≥30 days and immunosuppressant doses for ≥60 days. Glucocorticoid dose must be ≤60 mg/day prednisone during screening； 6. If a subject is receiving oral glucocorticoids alone, the dose should be at least 7.5 mg/day of prednisone (or an equivalent dose of other glucocorticoids). * For Systemic Sclerosis (SSc): 1. Diagnosed with diffuse cutaneous SSc per the 2013 EULAR/ACR criteria ； 2. Have positive ANA and/or SSc-related antibodies； 3. Disease duration ≤5 years (from initial diagnosis)； 4. Had an inadequate response or relapse after conventional therapy. Conventional therapy is defined as treatment with glucocorticoids plus any of the following immunomodulatory agents: cyclophosphamide, mycophenolate mofetil, methotrexate, leflunomide, azathioprine, tacrolimus, cyclosporine, and/or biologics (such as rituximab and belimumab), with a cumulative treatment duration of \>6 months； 5. Modified Rodnan Skin Score (mRSS) ≥15 and ≤30 at screening with progression within 6 months； 6. Prior to the first dose, subjects must have received glucocorticoids and/or immunosuppressants for ≥12 weeks, with stable glucocorticoid doses for ≥30 days and immunosuppressant doses for ≥60 days. The glucocorticoid dose during the screening period should not exceed 10 mg/day of prednisone (or an equivalent dose of other glucocorticoids). * Females of childbearing potential must use highly effective contraception from screening until 6 months after the last dose, refrain from oocyte donation, and ensure male partners use effective contraception. * Males of childbearing potential must use effective contraception from screening until 6 months after the last dose, with no plans for fertility or sperm donation, and ensure female partners use effective contraception. Exclusion Criteria: * Severe lupus nephritis within 8 weeks prior to screening (defined as urinary protein \>6 g/24 h, serum creatinine \>2.5 mg/dL or 221 μmol/L), requirement of prohibited medications for active nephritis per protocol, need for hemodialysis, or receipt of prednisone ≥100 mg/day (or equivalent glucocorticoids) for ≥14 days. * Central nervous system disorders (including but not limited to epilepsy, psychosis, interstitial encephalopathy syndrome, cerebrovascular accident, encephalitis, or CNS vasculitis) within 8 weeks prior to screening. * Other types of idiopathic inflammatory myopathies: inclusion body myositis, amyotrophic diabetes, or juvenile myositis; patients with severe muscle damage or permanent weakness/cardiac involvement due to non-IIM causes (e.g., stroke). * SSc-related pulmonary hypertension requiring treatment; rapidly progressive SSc-related lower gastrointestinal (small/large intestine) involvement requiring parenteral nutrition; active gastric antral vascular ectasia; history of SSc-related renal crisis. * History of major organ transplantation (e.g., heart, lung, kidney, liver) or hematopoietic stem cell/bone marrow transplantation. * Other concurrent autoimmune diseases requiring systemic therapy. * IgA deficiency (serum IgA level \<10 mg/dL). * Abnormal laboratory findings at screening: Liver function: AST/ALT \>2× upper limit of normal (ULN) (except for IIM patients