VISIONS AND PERSPECTIVES


ISJ 12: 1-4, 2015                                                                  ISSN 1824-307X 
 
 

VISIONS AND PERSPECTIVES 
 
Invertebrate immunological memory: could the epigenetic changes play the part of 
lymphocytes? 
 
E Ottaviani 
 
Department of Life Sciences, University of Modena and Reggio Emilia, Modena, Italy 
 
 

   Accepted December 10, 2014 

 
Abstract 
Different hypotheses have been suggested for the neurological memory storage in vertebrates, either 
based on the structural induction of synaptic plasticity or on chemical modifications, i.e., DNA 
rearrangement. For invertebrates, DNA rearrangements, and in particular the involvement of 
epigenetic mechanisms which in turn regulate gene expression, have been proposed. Based on the 
deep link existing among immune and neuroendocrine functions, it is argued here that epigenetic 
changes could represent the basis for explaining the numerous observations reporting hints of 
immunological memory in absence of lymphocytes. 
 

 

 
Introduction 

 
Memory is a wide term encompassing the 

storage of experiences into specific neurons and the 
trace of immune challenges into specific cells. 
Memorized experiences and pathogens are 
accumulated during the life (Rensing et al., 2009), 
but the fine mechanisms building up memory are 
still open to debate. 

Different hypotheses have been suggested for 
neurological memory (see for review, Peña de Ortiz 
and Arshavsky, 2001). One hypothesis proposes 
that the synaptic plasticity is the key event for 
memory consolidation, describing memory storage 
as the consequence of a structural modification. A 
second hypothesis considers a chemical 
modification, i.e., the memory storage is not at the 
synaptic, but at the genomic level. In this second 
view the storage of information is based on somatic 
DNA recombination. Such memory storage 
mechanism working in the brain neurons is similar 
to that observed in lymphocytes of the immune 
system. In this context, it is appropriate to stress the 
similarities between immune and neuroendocrine 
system workflow. Studies by J. Edwin Blalock and 
colleagues have demonstrated in mammals a 
functional integration of the immune and the 
neuroendocrine systems. One of the most 
convincing findings has been that both systems 
share a series of endogenous and exogenous 
___________________________________________________________________________ 

 
Corresponding author: 
Enzo Ottaviani 
Department of Life Sciences 
University of Modena and Reggio Emilia 
via Campi 213/D, 41125 Modena, Italy 
E-mail: enzo.ottaviani@unimore.it 

 

 
 
mediators that combat threats to the homeostasis of 
the organism (Weigent and Blalock, 1987). Based 
on these similarities, Habibi et al. (2009) surmized 
that DNA rearrangements could be present in 
immune and nervous systems in order to 
accumulate permanent information and allowing the 
creation of long-term memory. 

A possible relationship between genome 
changes and memory formation could be also 
contained into epigenetic modifications, i.e., 
alterations in the chromatin structure, which in turn 
regulate gene expression (Levenson and Sweatt, 
2006). 

The present contribution suggests to consider 
epigenetic modifications as one possible 
mechanism of immunological memory formation in 
invertebrates. 
 
Epigenetics 

The term epigenetics was coined by Conrad H. 
Waddington in 1942. Today, epigenetics has been 
defined as ‘‘the study of changes in gene function 
that are mitotically and/or meiotically heritable and 
that do not entail a change in DNA sequence’’ (Wu 
et al., 2001). The well-studied epigenetic 
mechanisms are the following: DNA methylation, 
histone tail modification and microRNA (miRNA) or 
non-coding RNA (Tammen et al., 2013) (Fig. 1). 
 
Current status on the immune and neuroendocrine 
systems in invertebrates 

Already 20 years ago, experiments of my and 
other laboratories have shown that in molluscs, as 
in vertebrates, there is a close functional correlation 
between the immune and the neuroendocrine 

1 
 



 
 
Fig. 1 Schematic summary of the epigenetic mechanisms acting on chromatin remodelling that are involved in 
gene regulation. 
 
 
 
 
 
systems. Given the evolutive distance separating 
mollusca form vertebrates, we concluded that this 
relationship has a deep and ancient evolutionary 
root, predating the split between protostomian and 
deuterostomian lineages. It seems that nature has 
followed an economic strategy by reusing the same 
pool of signal molecules (neuropeptides, hormones 
and cytokines) preserved in an extraordinary way in 
the course of evolution (Ottaviani and Franceschi, 
1997). 

With regards the immune system, invertebrates 
present only the innate immunity and consequently 
there are no lymphocytes challenging the 
comparative immunologists focused on 
immunological memory studies. Despite the lack of 
a cellular substrate recalling the vertebrate 
lymphocytes, in literature several data in supporting 
the presence of immunological memory emerge 
(Cooper, 1969, 1976; Hostetter and Cooper, 1973; 
Karp and Hildemann, 1976; Hildemann et al., 1977, 
1979a, b; Karp and Rheins, 1980). In our molluscan 
model, Planorbarius corneus, experiments 
performed on the humoral and cellular components, 

as well as on the bacterial clearance are in favor of 
the existence of some form of immunological 
memory (Ottaviani et al., 1986; Ottaviani, 1992). 
Repeated injections of bacteria Staphylococcus 
aureus and Escherichia coli into the foot of the 
mollusc induce specific and aspecific agglutinins. 
Specific agglutinins observed in direct-agglutination 
tests showed an increased titer after the second 
injection. Aspecific agglutinins evaluated in cross-
agglutination tests showed no changes in titer after 
the second injection (Ottaviani, 1992). The in vitro 
bacterial phagocytosis experiments have shown a 
higher bacterial elimination across the entire time-
range considered (30, 60, 90, 120, 150 min) in the 
snails that had already contacted the bacteria to be 
phagocytized (Ottaviani, 1992). The bacterial 
clearance experiments have revealed that after the 
second (14 days) and third (73 days) bacterial 
injections, clearance rates are faster (Ottaviani et 
al., 1986). Also more recent investigations have 
provided further evidence of a potential immune 
memory in invertebrates, though the edge between 
real memory and the effect of immune priming is 

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blurred, especially in short living species (Kurtz, 
2004, 2005; Brehélin and Roch, 2008). 

On the whole, a careful revision of the existing 
literature provides indications that from protozoans 
(Csaba et al., 1984) to vertebrates, some types of 
memory may be present. The absence of a 
lymphocyte-based system comparable to those of 
jawed vertebrates and lampreys (Hirano et al., 
2013), points towards alternative processes. The 
close developmental similarities recently observed 
between neurons and hemocytes in crustaceans 
(Benton et al., 2014) allow to speculate that the 
mechanisms described for neurological memory 
could be present also in invertebrates. With the 
exception of the data reported in Drosophila 
(LaFave and Sekelsky, 2009), mechanisms of 
somatic rearrangement of DNA are unusual and not 
occurring at loci hosting invertebrate immune genes. 
As an alternative mechanism, epigenetic 
modifications of hemocyte chromatin could be 
surmized, as it has been also proposed by 
Levenson and Sweatt (2006) for neurological 
memory. 

 
Epigenetic effects in invertebrates 

Recently, we have analyzed epigenetic 
modifications in neurons of the mollusc Pomacea 
canaliculata after injection of LPS (Ottaviani et al., 
2013). Following the treatment the phospho-
acetylation of histone H3 correlates with the 
increase of c-Fos protein levels in the nuclei of the 
small ganglionic neurons. These findings show the 
highly conserved interactions between immune and 
neuroendocrine systems at a molecular level. 

In contrast to the pattern of genome-wide DNA 
methylation in vertebrates, DNA methylation in 
invertebrates is relatively sparse (Bird et al., 1979; 
Suzuki and Bird, 2008; Field et al., 2004). 

Epigenetics studies in molluscs suggest the 
presence of CpG methylation in Mytilus edulis (Bird 
and Taggart, 1980), Donax trunculus (Petrovic et 
al., 2009) and Crassostrea gigas, where DNA 
methylation has important regulatory functions 
(Gavery and Roberts, 2010). Moreover, in Octopus 
vulgaris methylation is involved in gene regulation 
during the development (Díaz-Freije et al., 2014). 
Beside DNA methylation, miRNAs present in 
metazoan are about 22 nucleotides in length and 
play a role in the control of animal development and 
physiology by altering the chromatin architecture 
(Ambros, 2004).  

 
Conclusions 

 
This article wants to trigger the attention of 

comparative immunologists towards those 
mechanisms widespread in metazoans that could 
explain numerous observations at present orphans 
of solid explanations. At present it is neither 
possible to state that immunological memory is 
present in invertebrates nor that epigenetic changes 
represent its basis. However, the presence of 
epigenetic mechanisms could represent a potential 
alternative to the lymphocyte-based memory in 
invertebrates, especially in consideration of the very 
few examples of somatic DNA recombination. 
 

Acknowledgements 
The author thanks Prof. M Mandrioli (University 

of Modena and Reggio Emilia,  Italy) for the for the 
figure. 

 
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