SIV infection decreases sympathetic innervation of primate lymph nodes: the role of neurotrophins

Abstract

The sympathetic nervous system regulates immune responses in part through direct innervation of lymphoid organs. Recent data indicate that viral infections can alter the structure of lymph node innervation. To determine the molecular mechanisms underlying sympathetic denervation during Simian Immunodeficiency Virus (SIV) infection, we assessed the expression of neurotrophic factors and neuromodulatory cytokines within lymph nodes from experimentally infected rhesus macaques. Transcription of nerve growth factor (NGF), brain-derived neurotropic factor (BDNF) and neurotrophin-4 (NT4) decreased significantly in vivo during chronic SIV infection, whereas expression of the neuro-inhibitory cytokine interferon-gamma (IFN gamma) was up-regulated. Acute SIV infection of macaque leukocytes in vitro induced similar changes in the expression of neurotrophic and neuro-inhibitory factors, indicative of an innate immune response. Statistical mediation analyses of data from in vivo lymph node gene expression suggested that coordinated changes in expression of multiple neuromodulatory factors may contribute to SIV-induced depletion of catecholaminergic varicosities within lymphoid tissue. Given previous evidence that lymph node catecholaminergic varicosities can enhance SIV replication in vivo, these results are consistent with the hypothesis that reduced expression of neurotrophic factors during infection could constitute a neurobiological component of the innate immune response to viral infection.

Figure 1. Effect of SIV infection on lymph node sympathetic innervation

Parenchymal catecholaminergic varicosities (arrowheads) were localized in lymph node sections using glyoxylic acid chemofluorescence (a). Immune cells are counterstained red. Sites of innervation were mapped (yellow dots) onto whole lymph node sections from non-infected (b) and SIV-infected rhesus macaques (c). SIV-infection was confirmed by in situ hybridization against viral gag, pol, and env genes (c, inset). Spatial stereological techniques quantified the density of innervation in non-infected and SIV-infected lymph nodes (d).

Figure 2. Effect of SIV infection on neurotrophin mRNA expression in lymph nodes

Real-time RT-PCR quantification of neurotrophin mRNAs in lymph nodes biopsied from uninfected (-) and SIV-infected (+) macaques. mRNA levels have been normalized to expression of housekeeping genes.

Figure 3. Effect of SIV infection on neural inhibitor mRNA expression in lymph nodes

Expression of IFNG, LIF and SEMA3C mRNA in lymph nodes from non-infected (-) and SIV-infected (+) macaques was quantified by real-time RT-PCR. mRNA levels have been normalized to expression of housekeeping genes.

Figure 4. Effect of in vitro SIV infection on neurotrophin expression by leukocytes

Expression of NGF, BDNF, NT4 (a) and IFNG (b) in PBMC were quantified by real time RT-PCR. mRNA levels have been normalized to expression of housekeeping genes.

Figure 5. Model evaluating neuro-modulatory factors as mediators of SIV-induced denervation of lymphoid tissue

Top p-value evaluates statistical significance of SIV effects on innervation density. Bottom p-value indicates statistical significance of residual effect of SIV infection on innervation density after controlling for variations in postulated mediators NGF, BDNF, NT4, and IFNG.