Neurotrophins [1,2] are produced by immune organs and immunocompetent cells, including T cells  and macrophages , and are believed to play a role in various functions of the immune system, including lymphocyte proliferation [5,6]. Little is known about the effect of HIV-1 on neurotrophin levels. Loss of neurotrophin expression may impair the immune system and promote AIDS. In this study, we investigated whether HIV-1 reduces serum concentration of the neurotrophins and sought to establish a correlation between HIV infection and neurotrophin expression in T cells.
Serum levels of brain-derived neurotrophic factor (BDNF) were measured by an enzyme-linked immunosorbent assay in human samples collected between 1994 and 2007 at the Washington, District of Columbia site of the Women's Interagency HIV Study [7,8]. Because approximately 50% of these individuals were polydrug abusers, mainly cocaine, methamphetamine and heroin, a two-way analysis of variance (ANOVA) was used to examine a potential interaction between HIV-1 and drug use and to examine each factor independently. HIV-positive individuals exhibited significantly lower levels of BDNF compared with HIV-negative controls (Fig. 1a). Drug use significantly affected BDNF levels such that the amount of BDNF in the serum of HIV-positive drug users were higher than in HIV-positive nondrug users (Fig. 1a), suggesting that polydrug use may affect serum BDNF levels in HIV-1-positive individuals. There was no interaction between drug use and serostatus on BDNF levels (P > 0.33).
Drugs of abuse  or HIV-1 may influence the expression of other neurotrophins. To test this hypothesis, we measured nerve growth factor (NGF) and neurotrophin-3 (NT-3) levels in the same samples. The two-way ANOVAs analyzing associations of HIV status and drug use on NGF (P = 0.516) and NT-3 (P = 0.382) were not statistically significant, and no evidence of interaction between HIV and drug use was observed for either outcome. Although we found a tendency toward lower average NGF levels in the serum of HIV-positive individuals compared with controls, the effect was not significant (P = 0.89) nor did polydrug use affect NGF levels (data not shown). Results for NT-3 levels were similarly not statistically significant (data not shown).
The reduction of BDNF observed in HIV-1-positive individuals could be due to single nucleotide polymorphisms (SNPs) that alter intracellular packaging and secretion of BDNF . rs6265 is a polymorphism in the BDNF gene that produces an amino acid substitution of valine to methionine in codon 66 (Val66Met); rs56164415 is located in the fifth of the seven noncoding exons of the BDNF gene  and appears to be moderately associated with substance abuse . Therefore, these SNPs, either alone or in combination, might lead to a reduction in serum BDNF levels. To test this hypothesis, we examined the frequency of these polymorphisms in the same cohort, using DNA from the same sample of individuals. There was no significant difference in frequency of alleles in HIV individuals as compared with HIV-negative controls (rs6265, P = 0.83; rs56164415, P = 0.72). Therefore, mutation of the BDNF gene does not appear to account for difference in the levels of BDNF in these individuals.
Contributing factors that may account for the decrease in serum BDNF in HIV-positive individuals are not easily defined. BDNF and other neurotrophins are produced by immune organs and immunocompetent cells , as well as platelets . Thus, a decrease in the number of platelets may explain the lower levels of BDNF in HIV-1-positive individuals. To determine whether BDNF from platelets constitutes a significant fraction of serum BDNF, we examined which blood cell type expresses BDNF. We found that platelets and T cells exhibited comparable levels of BDNF expression (Fig. 1b). Thus, platelets account for only for a fraction of serum BDNF. Nevertheless, to more directly examine the effect of HIV-1 on BDNF, we examined the ability of HIV-1 to decrease BDNF expression in T cells. T lymphocytes were prepared from healthy donors and were infected with X4 (IIIB) or R5 (BaL) HIVs. BDNF mRNA levels were then quantified 24 h after the infection. We observed an approximately 50% decrease in BDNF mRNA levels by both HIV-1 strains (Fig. 1c), further suggesting that HIV-1 is capable of reducing the expression of this neurotrophin in T cells.
Our main finding is that the serum of HIV-positive women is characterized by reduced levels of BDNF, but not of NGF or NT-3, irrespective of drug use status, suggesting that HIV-1 influences the expression of selected neurotrophins. This was confirmed by direct evidence that both R5 and X4 HIV-1 strains downregulate BDNF mRNA levels in T cells. These results may contribute new insights into our understanding of the immune dysregulation of AIDS. In fact, given the well known antiapoptotic effect of the neurotrophins for T cells [6,13], we may speculate that a decrease in BDNF could be among the mechanisms employed by HIV-1 to induce apoptosis of T cells. On the contrary, experimental evidence has shown an inverse correlation between levels of BDNF and CXCR4  and CCR5  expression. These coreceptors are crucial for HIV-1 infection . Therefore, reduced levels of BDNF may be a risk factor for increasing HIV infection.
HIV-1 also causes axonal injury, neuronal loss and dementia . BDNF is critical for neuronal survival . Blood neurotrophin levels have been used to investigate the role of the neurotrophins in the pathogenesis of various neurodegenerative diseases. In fact, recent data have shown a relationship between BDNF in blood and Alzheimer's disease  and age-related cognitive impairment . Therefore, serum BDNF could be a predictor of risk for the development of neurological signs in HIV-positive individuals. Our findings of an association between HIV infection and serum BDNF levels, and of lowered BDNF mRNA levels in infected T cells, provide initial evidence in support of this hypothesis and suggest this neurotrophin as a possible biomarker for HIV dementia. Additional studies are needed to validate our results and extend them to both sexes, as we examined a relatively small cohort of women individuals. Also, a link between BDNF and cognitive performance needs to be established.
This study is supported by HHS grants DA026174 (I.M.), NS066842 (A.G.-D.). Women's Interagency HIV Study is funded by UO1-AI-35004, UO1-AI-31834, UO1-AI-34994, UO1-AI-34989, UO1-AI-34993, UO1-AI-42590, UO1-HD-32632 and UL1 RR024131.
1. Laurenzi MA, Barbany G, Timmusk T, Lindgren JA, Persson H. Expression of mRNA encoding neurotrophins and neurotrophin receptors in rat thymus, spleen tissue and immunocompetent cells. Regulation of neurotrophin-4 mRNA expression by mitogens and leukotriene B4. Eur J Biochem 1994; 223:733–741.
2. Artico M, Bronzetti E, Felici LM, Alicino V, Ionta B, Bronzetti B, et al
. Neurotrophins and their receptors in human lingual tonsil: an immunohistochemical analysis. Oncol Rep 2008; 20:1201–1206.
3. Kerschensteiner M, Gallmeier E, Behrens L, Leal VV, Misgeld T, Klinkert WE, et al
. Activated human T cells, B cells, and monocytes produce brain-derived neurotrophic factor in vitro and in inflammatory brain lesions: a neuroprotective role of inflammation? J Exp Med 1999; 189:865–870.
4. Elkabes S, DiCicco-Bloom EM, Black IB. Brain microglia/macrophages express neurotrophins that selectively regulate microglial proliferation and function. J Neurosci 1996; 16:2508–2521.
5. Aloe L, Bracci-Laudiero L, Micera A, Tirassa P. Nerve growth factor and the immune system. In: Mocchetti I, editor. Neurobiology of the neurotrophins
. Johnson City: Graham FP; 2005. pp. 237–253.
6. Garcia-Suarez O, Blanco-Gelaz MA, Lopez ML, Germana A, Cabo R, Diaz-Esnal B, et al
. Massive lymphocyte apoptosis in the thymus of functionally deficient TrkB mice. J Neuroimmunol 2002; 129:25–34.
7. Barkan SE, Melnick SL, Preston-Martin S, Weber K, Kalish LA, Miotti P, et al
. The Women's Interagency HIV Study. WIHS Collaborative Study Group. Epidemiology 1998; 9:117–125.
8. Bacon MC, von Wyl V, Alden C, Sharp G, Robison E, Hessol N, et al
. The Women's Interagency HIV Study: an observational cohort brings clinical sciences to the bench. Clin Diagn Lab Immunol 2005; 12:1013–1019.
9. Angelucci F, Ricci V, Pomponi M, Conte G, Mathe AA, Attilio Tonali P, Bria P. Chronic heroin and cocaine abuse is associated with decreased serum concentrations of the nerve growth factor and brain-derived neurotrophic factor. J Psychopharmacol 2007; 21:820–825.
10. Chen ZY, Patel PD, Sant G, Meng CX, Teng KK, Hempstead BL, Lee FS. Variant brain-derived neurotrophic factor (BDNF) (Met66) alters the intracellular trafficking and activity-dependent secretion of wild-type BDNF in neurosecretory cells and cortical neurons. J Neurosci 2004; 24:4401–4411.
11. Kunugi H, Ueki A, Otsuka M, Isse K, Hirasawa H, Kato N, et al
. A novel polymorphism of the brain-derived neurotrophic factor (BDNF) gene associated with late-onset Alzheimer's disease. Mol Psychiatry 2001; 6:83–86.
12. Liu QR, Walther D, Drgon T, Polesskaya O, Lesnick TG, Strain KJ, et al
. Human brain derived neurotrophic factor (BDNF) genes, splicing patterns, and assessments of associations with substance abuse and Parkinson's disease. Am J Med Genet B Neuropsychiatr Genet 2005; 134:93–103.
13. Vega JA, Garcia-Suarez O, Hannestad J, Perez-Perez M, Germana A. Neurotrophins and the immune system. J Anat 2003; 203:1–19.
14. Yamamoto H, Gurney ME. Human platelets contain brain-derived neurotrophic factor. J Neurosci 1990; 10:3469–3478.
15. Nosheny RL, Amhed F, Yakovlev AG, Meyer EM, Ren K, Tessarollo L, Mocchetti I. Brain-derived neurotrophic factor prevents the nigrostriatal degeneration induced by human immunodeficiency virus-1 glycoprotein 120 in vivo. Eur J Neurosci 2007; 25:2275–2284.
16. Ahmed F, Tessarollo L, Thiele C, Mocchetti I. Brain-derived neurotrophic factor modulates expression of chemokine receptors in the brain. Brain Res 2008; 1227:1–11.
17. Berger EA, Murphy PM, Farber JM. Chemokine receptors as HIV-1 coreceptors: roles in viral entry, tropism, and disease. Annu Rev Immunol 1999; 17:657–700.
18. Price RW. Neurological complications of HIV infection. Lancet 1996; 348:445–452.
19. Reichardt LF. Neurotrophin-regulated signalling pathways. Philos Trans R Soc Lond B Biol Sci 2006; 361:1545–1564.
20. Laske C, Stransky E, Leyhe T, Eschweiler GW, Maetzler W, Wittorf A, et al
. BDNF serum and CSF concentrations in Alzheimer's disease, normal pressure hydrocephalus and healthy controls. J Psychiatr Res 2007; 41:387–394.
21. Komulainen P, Pedersen M, Hanninen T, Bruunsgaard H, Lakka TA, Kivipelto M, et al
. BDNF is a novel marker of cognitive function in ageing women: the DR's EXTRA Study. Neurobiol Learn Mem 2008; 90:596–603.
22. Garzino-Demo A, Moss RB, Margolick JB, Cleghorn F, Sill A, Blattner WA, et al
. Spontaneous and antigen-induced production of HIV-inhibitory beta-chemokines are associated with AIDS-free status. Proc Natl Acad Sci USA 1999; 96:11986–11991.