Objectives Prior studies demonstrated associations between reduced serum 25-hydroxyvitamin D (25OHD), inflammation and disease activity in paediatric systemic lupus erythematosus (pSLE). intima press thickness or high-density lipoprotein (HDL) or low-density lipoprotein (LDL)?cholesterol, but was negatively associated with lipoprotein(a) levels. Despite no association with serum 25OHD, LL-37 was negatively associated with total cholesterol, HDL and LDL?cholesterol and positively associated with age. There was no significant difference in Z-DEVD-FMK biological activity mean LL-37 levels in participants with reported illness as an adverse event during the 3-yr APPLE study. Conclusions Despite links to vitamin D levels in other studies, LL-37 levels were not associated with baseline serum 25OHD concentrations in paediatric?individuals with pSLE. Despite the lack of correlation with 25OHD, LL-37 levels in this study were associated with cholesterol levels. Some subjects with pSLE have significantly elevated levels of LL-37 of unknown significance. Z-DEVD-FMK biological activity These exploratory results addressing the role of LL-37 levels in pSLE appear worthy of future study. demonstrated that an increase in serum vitamin D levels by 20 ng/mL (up to 40 ng/mL) was associated with a 21% decrease in the odds of having a high activity score.4 Prior studies using data from the Atherosclerosis Prevention in Paediatric Lupus Erythematosus (APPLE) cohort showed a statistically significant inverse relationship between serum 25-hydroxyvitamin D (25OHD) levels and high sensitivity C-reactive protein (hsCRP), suggesting that vitamin D deficiency is associated with heightened inflammation seen in patients?with pSLE.3 Human cathelicidin (LL-37) is an antimicrobial peptide (AMP) that belongs to a class of proteins present in mammals, called cathelicidins, which may confer an important role in innate immunity.5 LL-37 is a 37 amino acid protein, derived from a precursor molecule called human cathelicidin antimicrobial protein (hCAP18) and is the only form of cathelicidin found in humans.6 Secreted LL-37 is found at many barrier sites, typically epithelial surfaces throughout the body, including the skin, gastrointestinal (GI) tract, testis and respiratory tract.7 LL-37 is mainly upregulated when disease is present, especially bacterial, fungal and viral infections. It has been hypothesised that LL-37 may function by interacting with the microbial membrane, thereby disrupting the membranes normal activity and serving as an antimicrobial.8 Vitamin D is a known regulator of LL-37 in humans. Vitamin D administered in vitro in human monocytes results in the increased expression of hCAP18 and production of the protein LL-37 due to activation of a vitamin D response element (VDRE).9 10 Further, oral vitamin D therapy has been shown to increase the expression of hCAP18 in peripheral monocytes cultured ex vivo from adults with HIV and osteoporosis.4 11 Lowered expression of vitamin D or low levels of serum vitamin D have been linked to impaired immune response.12 This suggests a relationship between lowered LL-37 levels and deficient immune response. The hypothesis of this study was that vitamin D Rtn4rl1 status was associated with circulating serum LL-37 and parathyroid hormone (PTH) concentrations in children with pSLE in the Atherosclerosis Prevention in Paediatric Lupus Erythematosus (APPLE) trial. Additionally, we examined the relationship of LL-37, infection and markers of inflammation in patients with?pSLE. Cardiovascular markers which had previously been examined demonstrated a correlation between 25OHD deficiency and CRP and low-density lipoprotein (LDL) cholesterol,3 so we hypothesise that PTH will also have a correlation with these same factors due to the known relationship between 25OHD and PTH. Methods The APPLE study The design and methods of the APPLE trial have been reported previously.13 Briefly, APPLE was a prospective, randomised, placebo-controlled trial, enrolling 221 participants with paediatric-onset SLE (1997 American College of Rheumatology criteria) from 21 North American centers.14 15 Patients were excluded if indeed they got nephrotic syndrome, liver disease, renal insufficiency, hypercholesterolaemia (total cholesterol 350 mg/dL), myositis or were pregnant or lactating.13 Data were collected on disease activity (SLEDAI), disease harm (Systemic Lupus International Collaborating Treatment centers Damage Index (SLICC)) and relevant laboratory data. Grade 3 and above National Malignancy Institute Common Terminology Requirements for Adverse Occasions (CTCAE) Adverse Occasions, including disease, were collected through the 3-yr trial. Reviews of disease were sectioned off into bacterial, viral and fungal. Serum evaluation Frozen serum samples gathered at baseline of the APPLE trial had been available Z-DEVD-FMK biological activity for evaluation. 25OHD, PTH and LL-37 amounts were measured. 25OHD was measured.